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The grammar already until-now-incorrectly claimed that 'case A, B' worked, but insert an example now that it actually works. Swift SVN r8515
3786 lines
150 KiB
HTML
3786 lines
150 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Swift Language Reference Manual</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="author" content="Chris Lattner">
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<meta name="description"
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content="Swift Language Reference Manual.">
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<link rel="stylesheet" href="_static/swift.css" type="text/css">
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<script type="text/javascript" src="toc.js"></script>
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</head>
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<body>
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<h1>Swift Language Reference</h1>
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<p>
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<!-- The Table of Contents is automatically inserted in this <div>.
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Do not delete this <div>. -->
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<div id="nav"></div>
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</p>
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<!-- ********************************************************************* -->
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<h2>Introduction</h2>
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<!-- ********************************************************************* -->
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<div class="commentary">
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In addition to the main spec, there are lots of open ended questions,
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justification, and ideas of what best practices should be. That random
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discussion is placed in boxes to the right side of the main text (like this
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one) to clarify what is normative and what is discussion.
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</div>
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<p>This is the language reference manual for the Swift language, which is
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highly volatile and constantly under development. As the prototype evolves,
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this document should be kept up to date with what is actually implemented.</p>
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<p>The grammar and structure of the language is defined in BNF form in yellow
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boxes. Examples are shown in gray boxes, and assume that the standard library
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is in use (unless otherwise specified).</p>
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<!-- ===================================================================== -->
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<h3>Basic Goals</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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A non-goal of the Swift project in general is to become some amazing
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research project. We really want to focus on delivering a real product,
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and having the design and spec co-evolve.
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</div>
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<p>In no particular order, and not explained well:</p>
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<ol>
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<li>Support building great frameworks and applications, with a specific focus
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on permiting rich and powerful APIs.</li>
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<li>Get the defaults right: this reduces the barrier to entry and increases
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the odds that the right thing happens.</li>
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<li>Through our support for building great APIs, we aim to provide an
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expressive and productive language that is fun to program in.</li>
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<li>Support low-level system programming. We should want to write compilers,
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operating system kernels, and media codecs in Swift. This means that being
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able to obtain high performance is really quite important.</li>
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<li>Provide really great tools, like an IDE, debugger, profiling, etc.</li>
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<li>Where possible, steal great ideas instead of innovating new things that
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will work out in unpredictable ways. It turns out that there are a lot
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of good ideas already out there.</li>
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<li>Memory safe by default: array overrun errors, uninitialized values,
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and other problems endemic to C should not occur in Swift, even if it
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means some amount of runtime overhead. Eventually these checks will be
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disablable for people who want ultimate performance in production
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builds.</li>
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<li>Efficiently implementable with a static compiler: runtime compilation is
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great technology and Swift may eventually get a runtime optimizer, but it
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is a strong goal to be able to implement swift with just a static
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compiler.</li>
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<li>Interoperate as transparently as possible with C, Objective-C, and
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C++ without having to write an equivalent of "extern C" for every
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referenced definition.</li>
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<li>Great support for efficient by-value types.</li>
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<li>Elegant and natural syntax, aiming to be familiar and easy to transition
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to for "C" people. Differences from the C family should only be done when
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it provides a significant win (e.g. eliminate declarator syntax).</li>
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<li>Lots of other stuff too.</li>
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</ol>
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<p>A smaller wishlist goal is to support embedded sub-languages in swift, so
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that we don't get the OpenCL-is-like-C-but-very-different-in-many-details
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problem.</p>
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<!-- ===================================================================== -->
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<h3>Basic Approach</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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Pushing as much of the language as realistic out of the compiler and into
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the library is generally good for a few reasons: 1) we end up with a smaller
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core language. 2) we force the language that is left to be highly
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expressive and extensible. 3) this highly expressive language core can then
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be used to build a lot of other great libraries, hopefully many we can't
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even anticipate at this point.
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</div>
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<p>The basic approach in designing and implementing the Swift prototype was to
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start at the very bottom of the stack (simple expressions and the trivial
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bits of the type system) and incrementally build things up one brick at a
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time. There is a big focus on making things as simple as possible and
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having a clean internal core. Where it makes sense, sugar is added on top
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to make the core more expressive for common situations.</p>
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<p>One major aspect that dovetails with expressivity, learnability, and focus
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on API development is that much of the language is implemented in a <a
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href="#stdlib">standard library</a> (inspired in part by the Haskell
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Standard Prelude). This means that things like 'Int' and 'Void' are not
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part of the language itself, but are instead part of the standard library.
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</p>
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<!-- ********************************************************************* -->
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<h2>Phases of Translation</h2>
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<!-- ********************************************************************* -->
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<div class="commentary">
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Because Swift doesn't rely on a C-style "lexer hack" to know what is a type
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and what is a value, it is possible to fully parse a file without resolving
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import declarations.
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</div>
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<p>Swift has a strict separation between its phases of translation, and the
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compiler follows a conceptually simple design. The phases of translation
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are:</p>
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<ul>
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<li><a href="#lexical">Lexing</a>: A translation unit is broken into tokens
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according to a (nearly, /**/ comments can be nested) regular grammar.</li>
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<li>Parsing and AST Building: The tokens are parsed according to the grammar
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set out below. The grammar is context free and does not require any "type
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feedback" from the lexer or later stages. During parsing, name binding for
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references to local variables and other declarations that are not at
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translation unit (and eventually namespace) scope are bound.
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</li>
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<li><a href="#namebind">Name Binding</a>: At this phase, references to
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non-local types and values are bound, and <a href="#decl-import">import
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directives</a> are both validated and searched. Name binding can cause
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recursive compilation of modules that are referenced but not yet built.
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</li>
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<li><a href="#typecheck">Type Checking</a>: During this phase all types are
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resolved within value definitions, <a href="#expr-call">function
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application</a> and <a href="#expr-infix">binary expressions</a> are found
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and formed, and overloaded functions are resolved.</li>
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<li>Code Generation: The AST is converted the LLVM IR, optimizations are
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performed, and machine code generated.</li>
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<li>Linking: runtime libraries and referenced modules are linked in.</li>
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</ul>
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<p>
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FIXME: "import swift" implicitly added as the last import in translation unit.
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</p>
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<!-- ********************************************************************* -->
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<h2 id="lexical">Lexical Structure</h2>
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<!-- ********************************************************************* -->
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<div class="commentary">
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Not all characters are "taken" in the language, this is because it is still
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growing. As there becomes a reason to assign things into the identifier or
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punctuation bucket, we will do so as swift evolves.
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</div>
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<p>The lexical structure of a Swift file is very simple: the files are
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tokenized according to the following productions and categories. As is
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usual with most languages, tokenization uses the maximal munch rule and
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whitespace separates tokens. This means that "a b" and "ab" lex into
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different token streams and are therefore different in the grammar.</p>
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<!-- ===================================================================== -->
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<h3 id="whitespace">Whitespace and Comments</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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Nested block comments are important because we don't have the nestable
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"#if 0" hack from C to rely on.
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</div>
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<pre class="grammar">
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whitespace ::= ' '
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whitespace ::= '\n'
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whitespace ::= '\r'
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whitespace ::= '\t'
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whitespace ::= '\0'
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comment ::= //.*[\n\r]
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comment ::= /* .... */
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</pre>
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<p>Space, newline, tab, and the nul byte are all considered whitespace and are
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discarded, with one exception: a '(' or '[' which does not follow a
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non-whitespace character is different kind of token (called
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<em>spaced</em>) from one which does not (called <em>unspaced</em>).
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A '(' or '[' at the beginning of a file is spaced.</p>
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<p>Comments may follow the BCPL style, starting with a "//" and running to the
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end of the line, or may be recursively nested /**/ style comments. Comments
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are ignored and treated as whitespace.</p>
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<!-- ===================================================================== -->
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<h3 id="reserved_punctuation">Reserved Punctuation Tokens</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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The difference between reserved punctuation and identifiers is that you
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can't "overload an operator" with one of these names.<br><br>
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Note that -> is used for function types "() -> Int", not pointer
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dereferencing.
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</div>
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<pre class="grammar">
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punctuation ::= '('
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punctuation ::= ')'
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punctuation ::= '{'
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punctuation ::= '}'
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punctuation ::= '['
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punctuation ::= ']'
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punctuation ::= '.'
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punctuation ::= ','
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punctuation ::= ';'
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punctuation ::= ':'
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punctuation ::= '='
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punctuation ::= '->'
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punctuation ::= '...'
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punctuation ::= '&' // unary prefix operator
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</pre>
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<p>These are all reserved punctuation that are lexed into tokens. Most other
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punctuation is matched as <a href="#identifier">identifiers</a>.
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</p>
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<!-- ===================================================================== -->
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<h3>Reserved Keywords</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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The number of keywords is reduced by pushing most functionality
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into the library (e.g. "builtin" datatypes like 'Int' and 'Bool'). This
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allows us to add new stuff to the library in the future without worrying
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about conflicting with the user's namespace.
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</div>
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<pre class="grammar">
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// Declarations and Type Keywords
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keyword ::= 'class'
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keyword ::= 'destructor'
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keyword ::= 'extension'
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keyword ::= 'import'
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keyword ::= 'init'
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keyword ::= 'func'
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keyword ::= 'metatype'
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keyword ::= 'enum'
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keyword ::= 'protocol'
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keyword ::= 'static'
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keyword ::= 'struct'
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keyword ::= 'subscript'
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keyword ::= 'typealias'
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keyword ::= 'var'
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keyword ::= 'where'
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// Statements
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keyword ::= 'break'
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keyword ::= 'case'
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keyword ::= 'continue'
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keyword ::= 'default'
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keyword ::= 'do'
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keyword ::= 'else'
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keyword ::= 'if'
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keyword ::= 'in'
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keyword ::= 'for'
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keyword ::= 'return'
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keyword ::= 'switch'
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keyword ::= 'then'
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keyword ::= 'while'
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// Expressions
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keyword ::= 'as'
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keyword ::= 'is'
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keyword ::= 'new'
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keyword ::= 'super'
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keyword ::= 'self'
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keyword ::= 'Self'
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keyword ::= '__COLUMN__'
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keyword ::= '__FILE__'
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keyword ::= '__LINE__'
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</pre>
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<p>These are the builtin keywords.</p>
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<!-- ===================================================================== -->
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<h3 id="integer_literal">Integer Literals</h3>
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<!-- ===================================================================== -->
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<pre class="grammar">
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integer_literal ::= [0-9][0-9_]*
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integer_literal ::= 0x[0-9a-fA-F][0-9a-fA-F_]*
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integer_literal ::= 0o[0-7][0-7_]*
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integer_literal ::= 0b[01][01_]*
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</pre>
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<p>Integer literal tokens represent simple integer values of unspecified
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precision. They may be expressed in decimal, binary with the '<tt>0b</tt>'
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prefix, octal with the '<tt>0o</tt>' prefix, or hexadecimal with the
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'<tt>0x</tt>' prefix. Unlike C, a leading zero does not affect the base
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of the literal.
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<p>Integer literals may contain underscores at arbitrary positions after
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the first digit. These underscores may be used for human readability
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and do not affect the value of the literal.</p>
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<pre class="example">
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789
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0789
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1000000
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1_000_000
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0b111_101_101
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0o755
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0b1111_1011
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0xFB
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</pre>
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<!-- ===================================================================== -->
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<h3 id="floating_literal">Floating Point Literals</h3>
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<!-- ===================================================================== -->
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<div class="commentary">
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We require a digit on both sides of the dot to allow lexing "4.km" as
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"4 . km" instead of "4. km" and for a series of dots to be an operator (for
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ranges). The regex for decimal literals is same as Java, and the one for
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hex literals is the same as C99, except that we do not allow a trailing
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suffix that specifies a precision.
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</div>
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<pre class="grammar">
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floating_literal ::= [0-9][0-9_]*\.[0-9][0-9_]*
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floating_literal ::= [0-9][0-9_]*\.[0-9][0-9_]*[eE][+-]?[0-9][0-9_]*
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floating_literal ::= [0-9][0-9_]*[eE][+-]?[0-9][0-9_]*
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floating_literal ::= 0x[0-9A-Fa-f][0-9A-Fa-f_]*
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(\.[0-9A-Fa-f][0-9A-Fa-f_]*)?[pP][+-]?[0-9][0-9_]*
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</pre>
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<p>Floating point literal tokens represent floating point values of
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unspecified precision. Decimal and hexadecimal floating-point literals are
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supported.
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<p>The integer, fraction, and exponent of a floating point literal may each
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contain underscores at arbitrary positions after their first digits. These
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underscores may be used for human readability and do not affect the value
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of the literal. Each part of the floating point literal must however start
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with a digit; <tt>1._0</tt> would be a reference to the <tt>_0</tt> member
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of <tt>1</tt>.
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<pre class="example">
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1.0
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1000000.75
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1_000_000.75
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0x1.FFFFFFFFFFFFFp1022
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0x1.FFFF_FFFF_FFFF_Fp1_022
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</pre>
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<!-- ===================================================================== -->
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<h3 id="character_literal">Character Literals</h3>
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<!-- ===================================================================== -->
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<pre class="grammar">
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character_literal ::= '[^'\\\n\r]|character_escape'
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character_escape ::= [\]0 [\][\] | [\]t | [\]n | [\]r | [\]" | [\]'
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character_escape ::= [\]x hex hex
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character_escape ::= [\]u hex hex hex hex
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character_escape ::= [\]U hex hex hex hex hex hex hex hex
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hex ::= [0-9a-fA-F]
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</pre>
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<p>character_literal tokens represent a single character, and are surrounded
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by single quotes.</p>
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<p>The ASCII and Unicode character escapes:</p>
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<pre>
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\0 == nul
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\n == new line
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\r == carriage return
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\t == horizontal tab
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\u == small Unicode code points
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\U == large Unicode code points
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\x == raw ASCII byte (less than 0x80)
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</pre>
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<!-- ===================================================================== -->
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<h3 id="string_literal">String Literals</h3>
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<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
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FIXME: Forcing + to concatenate strings is somewhat gross, a proper protocol
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would be better.
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</div>
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<pre class="grammar">
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string_literal ::= ["]([^"\\\n\r]|character_escape|escape_expr)*["]
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escape_expr ::= [\]escape_expr_body
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escape_expr_body ::= [(]escape_expr_body[)]
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escape_expr_body ::= [^\n\r"()]
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</pre>
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<p>string_literal tokens represent a string, and are surrounded by double
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quotes. String literals cannot span multiple lines.</p>
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<p>String literals may contain embedded expressions in them (known as
|
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"interpolated expressions") subject to some specific lexical constraints:
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the expression may not contain a double quote ["], newline [\n], or carriage
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return [\r]. All parentheses must be balanced.</p>
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<p>In addition to these lexical rules, an interpolated expression must satisfy
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the <a href="#expr">expr</a> production of the general swift grammar. This
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expression is evaluated, and passed to the constructor for the inferred type
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of the string literal. It is concatenated onto any fixed portions of the
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|
string literal with a global "+" operator that is found through normal name
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lookup.</p>
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<pre class="example">
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// Simple string literal.
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"Hello world!"
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// Interpolated expressions.
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"\(min)..\(max)" + "Result is \((4+i)*j)"
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</pre>
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|
|
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<!-- ===================================================================== -->
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<h3 id="identifier">Identifier Tokens</h3>
|
|
<!-- ===================================================================== -->
|
|
|
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<div>
|
|
<pre class="grammar">
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identifier ::= id-start id-continue*
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|
|
|
// An identifier can start with an ASCII letter or underscore...
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id-start ::= [A-Za-z_]
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// or a Unicode alphanumeric character in the Basic Multilingual Plane...
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// (excluding combining characters, which can't appear initially)
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id-start ::= [\u00A8\u00AA\u00AD\u00AF\u00B2-\u00B5\u00B7-00BA]
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id-start ::= [\u00BC-\u00BE\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u00FF]
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id-start ::= [\u0100-\u02FF\u0370-\u167F\u1681-\u180D\u180F-\u1DBF]
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id-start ::= [\u1E00-\u1FFF]
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id-start ::= [\u200B-\u200D\u202A-\u202E\u203F-\u2040\u2054\u2060-\u206F]
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id-start ::= [\u2070-\u20CF\u2100-\u218F\u2460-\u24FF\u2776-\u2793]
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id-start ::= [\u2C00-\u2DFF\u2E80-\u2FFF]
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id-start ::= [\u3004-\u3007\u3021-\u302F\u3031-\u303F\u3040-\uD7FF]
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id-start ::= [\uF900-\uFD3D\uFD40-\uFDCF\uFDF0-\uFE1F\uFE30-FE44]
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id-start ::= [\uFE47-\uFFFD]
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// or a non-private-use, valid code point outside of the BMP.
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id-start ::= [\u10000-\u1FFFD\u20000-\u2FFFD\u30000-\u3FFFD\u40000-\u4FFFD]
|
|
id-start ::= [\u50000-\u5FFFD\u60000-\u6FFFD\u70000-\u7FFFD\u80000-\u8FFFD]
|
|
id-start ::= [\u90000-\u9FFFD\uA0000-\uAFFFD\uB0000-\uBFFFD\uC0000-\uCFFFD]
|
|
id-start ::= [\uD0000-\uDFFFD\uE0000-\uEFFFD]
|
|
|
|
// After the first code point, an identifier can contain ASCII digits...
|
|
id-continue ::= [0-9]
|
|
|
|
// and/or combining characters...
|
|
id-continue ::= [\u0300-\u036F\u1DC0-\u1DFF\u20D0-\u20FF\uFE20-\uFE2F]
|
|
|
|
// in addition to the starting character set.
|
|
id-continue ::= id-start
|
|
|
|
identifier-or-any ::= identifier
|
|
identifier-or-any ::= '_'
|
|
</pre>
|
|
|
|
<p>The set of valid identifier characters is consistent with WG14 N1518,
|
|
"Recommendations for extended identifier characters for C and C++".
|
|
This roughly corresponds to the alphanumeric characters in the Basic
|
|
Multilingual Plane and all non-private-use code points outside of the BMP.
|
|
It excludes mathematical symbols, arrows, line and
|
|
box drawing characters, and private-use and invalid code points.
|
|
An identifier cannot begin with one of the ASCII digits '0' through '9'
|
|
or with a combining character.</p>
|
|
|
|
<p>The Swift compiler does not normalize Unicode source code, and matches
|
|
identifiers by code points only. Source code must be normalized to a
|
|
consistent normalization form before being submitted to the compiler.
|
|
|
|
<pre class=example>
|
|
// Valid identifiers
|
|
foo
|
|
_0
|
|
swift
|
|
vernissé
|
|
闪亮
|
|
מבריק
|
|
😄
|
|
|
|
// Invalid identifiers
|
|
☃ // Is a symbol
|
|
0cool // Starts with an ASCII digit
|
|
́foo // Starts with a combining character
|
|
 // Is a private-use character
|
|
</pre>
|
|
|
|
<h4>Operators</h4>
|
|
|
|
<div class="commentary">
|
|
<br><br><b>[1]</b> The '=' token is explicitly handled in the grammar elsewhere,
|
|
and in general users cannot provide custom definitions for the '=' operator.
|
|
This distinctly differs from C++, which allows '=' to be overloaded.
|
|
|
|
<br><br><b>[2]</b> The '->' token is <a href="#reserved_punctuation">reserved punctuation</a>,
|
|
and cannot be used as an operator identifier.
|
|
|
|
<br><br><b>[3]</b> The unary prefix '&' token is <a href="#reserved_punctuation">reserved punctuation</a>,
|
|
and cannot be used as an operator identifier.
|
|
|
|
<br><br><b>[4]</b> The '//', '/*', and '*/' tokens are <a href="#whitespace">reserved for comments</a>,
|
|
and cannot be used as operator identifiers.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
<a name="operator">operator</a> ::= [@/=-+*%<>!&|^~]+
|
|
<a name="operator">operator</a> ::= \.\.
|
|
|
|
Note: excludes '=', see <b>[1]</b>
|
|
excludes '->', see <b>[2]</b>
|
|
excludes unary '&', see <b>[3]</b>
|
|
excludes '//', '/*', and '*/', see <b>[4]</b>
|
|
'..' is an operator, not two '.'s.
|
|
|
|
operator-binary ::= operator
|
|
operator-prefix ::= operator
|
|
operator-postfix ::= operator
|
|
|
|
left-binder ::= [ \r\n\t\(\[\{,;:]
|
|
right-binder ::= [ \r\n\t\)\]\},;:]
|
|
|
|
<a name="any-identifier">any-identifier</a> ::= identifier | operator
|
|
</pre>
|
|
|
|
<p><tt>operator-binary</tt>, <tt>operator-prefix</tt>, and
|
|
<tt>operator-postfix</tt> are distinguished by immediate lexical
|
|
context. An operator token is called <i>left-bound</i> if it
|
|
is immediately preceded by a character matching <tt>left-binder</tt>.
|
|
An operator token is called <i>right-bound</i> if it is immediately
|
|
followed by a character matching <tt>right-binder</tt>. An operator
|
|
token is an <tt>operator-prefix</tt> if it is right-bound but not
|
|
left-bound, an <tt>operator-postfix</tt> if it is left-bound but
|
|
not right-bound, and an <tt>operator-binary</tt> in either of the
|
|
other two cases.
|
|
</p>
|
|
|
|
<p>As an exception to the above rule, an operator immediately followed by a
|
|
dot ('.') is only considered right-bound if not already left-bound. This
|
|
allows <code>a@.prop</code> to be parsed as <code>(a@).prop</code> rather
|
|
than as <code>a @ .prop</code>. Similarly, because the '!' operator is
|
|
defined by the standard library to destructure
|
|
<a href="#type-optional">optional types</a> and is thus expected to be used
|
|
ubiquitously, it is also only considered right-bound if not already
|
|
left-bound.</p>
|
|
|
|
<p>When parsing certain grammatical constructs that involve '<' and
|
|
'>' (such as <a href="#type-composition">protocol composition
|
|
types</a>), an <tt>operator</tt> with a leading '<' or '>' may
|
|
be split into two or more tokens: the leading '<' or '>' and
|
|
the remainder of the token, which may be an <tt>operator</tt> or
|
|
<tt>punctuation</tt> token that may itself be further split. This
|
|
rule allows us to parse nested constructs such as
|
|
<code>A<B<C>></code> without requiring spaces between
|
|
the closing '>'s.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="dollarident">Implementation Identifier Token</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
dollarident ::= '$' id-continue+
|
|
</pre>
|
|
|
|
<p>Tokens that start with a $ are separate class of identifier, which are
|
|
fixed purpose names that are defined by the implementation.
|
|
</p>
|
|
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="decl">Declarations</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
decl ::= <a href="#decl-class">decl-class</a>
|
|
decl ::= <a href="#decl-constructor">decl-constructor</a>
|
|
decl ::= <a href="#decl-destructor">decl-destructor</a>
|
|
decl ::= <a href="#decl-extension">decl-extension</a>
|
|
decl ::= <a href="#decl-func">decl-func</a>
|
|
decl ::= <a href="#decl-import">decl-import</a>
|
|
decl ::= <a href="#decl-enum">decl-enum</a>
|
|
decl ::= <a href="#decl-enum-element">decl-enum-element</a>
|
|
decl ::= <a href="#decl-protocol">decl-protocol</a>
|
|
decl ::= <a href="#decl-struct">decl-struct</a>
|
|
decl ::= <a href="#decl-typealias">decl-typealias</a>
|
|
decl ::= <a href="#decl-var">decl-var</a>
|
|
decl ::= <a href="#decl-subscript">decl-subscript</a>
|
|
</pre>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-translation-unit">Translation Unit</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
translation-unit ::= <a href="#brace-item-list">brace-item</a>*
|
|
</pre>
|
|
|
|
<p>The top level of a swift source file is grammatically identical to the
|
|
contents of a func decl. Some declarations
|
|
have semantic restrictions that only allow them within a translation unit
|
|
though.
|
|
</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="brace-item-list">Brace Enclosed Items</h4>
|
|
|
|
<pre class="grammar">
|
|
brace-item-list ::= '{' brace-item* '}'
|
|
|
|
brace-item ::= <a href="#decl">decl</a>
|
|
brace-item ::= <a href="#expr">expr</a>
|
|
brace-item ::= <a href="#stmt">stmt</a>
|
|
</pre>
|
|
|
|
<p>The brace item list provides a sequencing operation which evaluates the
|
|
members of its body in order. Function bodies and the bodies of control
|
|
flow statements use braces. Also, the <a
|
|
href="#decl-translation-unit">translation unit</a> itself is effectively a
|
|
brace item list, but without the braces.
|
|
</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-import">import Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-import ::= 'import' <a href="#attribute-list">attribute-list</a> import-kind? import-path
|
|
|
|
import-kind ::= 'typealias'
|
|
import-kind ::= 'struct'
|
|
import-kind ::= 'class'
|
|
import-kind ::= 'enum'
|
|
import-kind ::= 'protocol'
|
|
import-kind ::= 'var'
|
|
import-kind ::= 'func'
|
|
|
|
import-path ::= <a href="#any-identifier">any-identifier</a> ('.' <a href="#any-identifier">any-identifier</a>)*
|
|
</pre>
|
|
|
|
<p>'import' declarations allow named values and types to be accessed with
|
|
local names, even when they are defined in other modules and namespaces. See
|
|
the section on <a href="#namebind">name binding</a> for more
|
|
information on how these work. import declarations are only allowed at
|
|
translation unit scope.</p>
|
|
|
|
<p>'import' directives only impact a single translation unit: imports in one
|
|
swift file do not affect name lookup in another file. import directives can
|
|
only occur at the top level of a file, not within a function or namespace.</p>
|
|
|
|
<p>An import without an explicit import-kind names a module; all of the
|
|
module's members are imported into the current scope. The module's name is
|
|
also imported into the the current scope in order to allow qualified access
|
|
to the module's members, which can be useful for disambiguation.</p>
|
|
|
|
<p>If an import-kind is provided, the last element of the import path is
|
|
taken to be the name of a decl <em>within</em> the module named by the rest of
|
|
the path. Only that name is introduced into the current scope; the name of the
|
|
module itself is <em>not</em> accessible, nor any other decls within the
|
|
module.</p>
|
|
|
|
<p>Different import-kinds perform different filters on the decls within a
|
|
module:</p>
|
|
|
|
<ul>
|
|
<li><code>typealias</code> can be used to import any concrete type (struct,
|
|
class, enum, or another typealias). It cannot be used to import protocols,
|
|
which are often used for more than just their existential type.</li>
|
|
|
|
<li><code>struct</code>, <code>class</code>, <code>enum</code> can be used
|
|
to import any type whose <i>canonical type</i> is a struct, class,
|
|
or enum, respectively. (This allows "Int" to be imported as a struct, for
|
|
example, even though its definition in the standard library may be a
|
|
typealias for another struct type.)</li>
|
|
|
|
<li><code>protocol</code> is used to import a protocol</li>
|
|
|
|
<li><code>var</code> is used to import a module-scoped variable</li>
|
|
|
|
<li><code>func</code> will import all overloads of a function</li>
|
|
</ul>
|
|
|
|
<pre class="example">
|
|
<i>// Import all of the top level symbols and types in a module.</i>
|
|
import swift
|
|
|
|
<i>// Import all of the symbols within a submodule.</i>
|
|
import swift.io
|
|
|
|
<i>// Import a single variable, function, type, etc.</i>
|
|
import typealias swift.io.BufferedStream
|
|
|
|
<i>// Import all addition overloads.</i>
|
|
import func swift.+
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-extension">extension Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-extension ::= 'extension' <a href="#type-identifier">type-identifier</a> <a href="#inheritance">inheritance</a>? '{' <a href="#decl">decl</a>* '}'
|
|
</pre>
|
|
|
|
<p>'extension' declarations allow adding member declarations to existing
|
|
types, even in other translation units and modules. There are different
|
|
semantic rules for each type that is extended.
|
|
</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="decl-extension-enum-struct"><a href="#decl-enum">enum</a>, <a
|
|
href="#decl-struct">struct</a>, and <a href="#decl-class">class</a>
|
|
declaration extensions</h4>
|
|
|
|
<p>FIXME: Write this section.</p>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-var">var Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-var ::= 'var' <a href="#attribute-list">attribute-list</a> <a href="#pattern">pattern</a> initializer? (',' pattern initializer?)*
|
|
|
|
decl-var ::= 'var' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> ':' <a href="#type">type-annotation</a> <a href="#brace-item-list">brace-item-list</a>
|
|
|
|
decl-var ::= 'var' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> ':' <a href="#type">type-annotation</a> '{' get-set '}'
|
|
|
|
initializer ::= '=' <a href="#expr">expr</a>
|
|
|
|
<a name="get-set"></a>get-set ::= get set?
|
|
get-set ::= set get
|
|
|
|
get ::= 'get:' <a href="#brace-item-list">brace-item*</a>
|
|
|
|
set ::= 'set' set-name? ':' <a href="#brace-item-list">brace-item*</a>
|
|
|
|
set-name ::= '(' <a href="#identifier">identifier</a> ')'
|
|
</pre>
|
|
|
|
|
|
<p>'var' declarations form the backbone of value declarations in Swift. A var
|
|
declaration takes a pattern and an optional initializer, and declares all the
|
|
pattern-identifiers in the pattern as variables. If there is an initializer
|
|
and the pattern is <a href="#fully_typed_types">fully-typed</a>, the
|
|
initializer is converted to the type of the pattern. If there is an
|
|
initializer and the pattern is not fully-typed, the type of initializer is
|
|
computed independently of the pattern, and the type of the pattern is derived
|
|
from the initializer. If no initializer is specified, the pattern must be
|
|
fully-typed, and the values are default-initialized.</p>
|
|
|
|
<p>If there is more than one pattern in a 'var' declaration, they are each
|
|
considered independently, as if there were multiple declarations. The
|
|
initial attribute-list is shared between all the declared variables.
|
|
|
|
<p>A var declaration may contain a getter and (optionally) a setter,
|
|
which will be used when reading or writing the variable, respectively.
|
|
Such a variable does not have any associated storage. A var
|
|
declaration with a getter or setter must have a type (call it
|
|
<code>T</code>). The getter function, whose body is provided as part
|
|
of the <code>var-get</code> clause, has type <code>() -> T</code>.
|
|
Similarly, the setter function, whose body is part of the
|
|
<code>var-set</code> clause (if provided), has type <code>(T)
|
|
-> ()</code>. If the <code>var-set</code> clause contains a <code>var-set-name</code>
|
|
clause, the identifier of that clause is used as the name of the
|
|
parameter to the setter. Otherwise, the parameter name is "value".</p>
|
|
|
|
<p>FIXME: Should the type of a pattern which isn't fully typed affect the
|
|
type-checking of the expression (i.e. should we compute a structured
|
|
dependent type)?</p>
|
|
|
|
<p>Like all other declarations, var's can optionally have a list of <a
|
|
href="#attribute-list">attributes</a> applied to them.</p>
|
|
|
|
<p>The type of a variable must be
|
|
<a href="#materializable"><i>materializable</i></a>. A variable is
|
|
an lvalue unless it has a <code>var-get</code> clause but not
|
|
<code>var-set</code> clause. </p>
|
|
|
|
<p>Here are some examples of var declarations:</p>
|
|
|
|
<pre class="example">
|
|
<i>// Simple examples.</i>
|
|
var a = 4
|
|
var b : Int
|
|
var c : Int = 42
|
|
|
|
<i>// This decodes the tuple return value into independently named parts</i>
|
|
<i>// and both 'val' and 'err' are in scope after this line.</i>
|
|
var (val, err) = foo()
|
|
|
|
<i>// Variable getter/setter</i>
|
|
var _x : Int = 0
|
|
var x_modify_count : Int = 0
|
|
var x1 : Int {
|
|
return _x
|
|
}
|
|
var x2 : Int {
|
|
get:
|
|
return _x
|
|
set:
|
|
x_modify_count = x_modify_count + 1
|
|
_x = value
|
|
}
|
|
</pre>
|
|
|
|
<p>Note that both 'get' and 'set' are context-sensitive keywords.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-func">func Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-func ::= 'static'? 'func' <a href="#attribute-list">attribute-list</a> <a href="#any-identifier">any-identifier</a> <a href="#generic-params">generic-params</a>? <a href="#func-signature">func-signature</a> <a href="#brace-item-list">brace-item-list</a>?
|
|
</pre>
|
|
|
|
<p>'func' is a declaration for a function. The argument list and
|
|
optional return value are specified by the type production of the function,
|
|
and the body is either a brace expression or elided. Like all other
|
|
declarations, functions are can have attributes.</p>
|
|
|
|
<p>If the type is not syntactically a function type (i.e., has no -> in it
|
|
at top-level), then the return value is implicitly inferred to be
|
|
"<tt>()</tt>". All of the argument and return value names are injected into
|
|
the <a href="#namebind_scope">scope</a> of the function body.</p>
|
|
|
|
<p>A function in an <a href="#decl-extension">extension</a> of some type (or
|
|
in other places that are semantically equivalent to an extension) implicitly
|
|
get a 'self' argument with these rules ... [todo]</p>
|
|
|
|
<p>'static' functions are only allowed in an <a
|
|
href="#decl-extension">extension</a> of some type (or in other places
|
|
that are semantically equivalent to an extension). They indicate that
|
|
the function is actually defined on the <a href="#metatype">metatype</a>
|
|
for the type, not on the
|
|
type itself. Thus it does not implicitly get a first 'self' argument, and
|
|
can be used with dot syntax on the metatype.</p>
|
|
|
|
<p>TODO: Func should be an immutable name binding, it should implicitly add
|
|
an attribute immutable when it exists.</p>
|
|
|
|
<p>TODO: Incoming arguments should be readonly, result should be implicitly
|
|
writeonly when we have these attributes.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="func-signature">Function signatures</h4>
|
|
|
|
<pre class="grammar">
|
|
func-signature ::= func-arguments func-signature-result?
|
|
func-arguments ::= <a href="#pattern-tuple">pattern-tuple</a>+
|
|
func-arguments ::= selector-tuple
|
|
<a id="selector-tuple">selector-tuple</a> ::= '(' <a href="#pattern-tuple">pattern-tuple-element</a> ')' (<a href="#identifier">identifier-or-any</a> '(' pattern-tuple-element ')')+
|
|
func-signature-result ::= '->' <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p>A function signature specifies one or more sets of parameter
|
|
patterns, plus an optional result type.</p>
|
|
|
|
<p>When a result type is not written, it is implicitly the empty tuple type,
|
|
<tt>()</tt>.</p>
|
|
|
|
<p>In the body of the function described by a particular signature,
|
|
all the variables bound by all of the parameter patterns are in
|
|
scope, and the function must return a value of the result type.</p>
|
|
|
|
<p>An outermost pattern in a function signature must be <a
|
|
href="#fully_typed_types">fully-typed</a> and irrefutable. If a result type is
|
|
given, it must also be fully-typed.</p>
|
|
|
|
<p>The type of a function with signature <tt>(P<sub>0</sub>)(P<sub>1</sub>)..(P<sub><i>n</i></sub>) -> R</tt>
|
|
is <tt>T<sub>0</sub> -> T<sub>1</sub> -> .. -> T<sub><i>n</i></sub> -> R</tt>,
|
|
where <tt>T<sub><i>i</i></sub></tt> is the bottom-up type of the pattern
|
|
<tt>P<sub><i>i</i></sub></tt>. This is called "currying". The
|
|
behavior of all the intermediate functions (those which do not
|
|
return <tt>R</tt>) is to capture their arguments, plus any
|
|
arguments from prior patterns, and returns a function which takes
|
|
the next set of arguments. When the "uncurried" function is
|
|
called (the one taking <tt>T<sub><i>n</i></sub></tt> and returning
|
|
<tt>R</tt>), all of the arguments are then available and the
|
|
function body is finally evaluated as normal.</p>
|
|
|
|
<p>A function declared with a selector-style signature
|
|
<tt>func(a<sub>0</sub>:T<sub>0</sub>) name<sub>1</sub>(a<sub>1</sub>:T<sub>1</sub>) .. name<sub><i>n</i></sub>(a<sub><i>n</i></sub>:T<sub><i>n</i></sub>) -> R</tt>
|
|
has the type <tt>(_:T<sub>0</sub>, name<sub>1</sub>:T<sub>1</sub>, .. name<sub><i>n</i></sub>:T<sub><i>n</i></sub>) -> R</tt>,
|
|
that is, the names of the fields in the argument tuple are the
|
|
<tt>name<sub><i>n</i></sub></tt> identifiers preceding each argument
|
|
pattern. However, in the body of a function
|
|
described by a signature, those arguments will be bound using the
|
|
corresponding
|
|
<tt>a<sub><i>n</i></sub></tt> patterns inside
|
|
the arguments. This allows for Cocoa-style keyword function
|
|
names such as <tt>doThing(x, withThing:y)</tt> to be defined without
|
|
requiring that an awkward keyword name be the same as the
|
|
variable name.
|
|
|
|
<p>Here are some examples of func definitions:</p>
|
|
|
|
<pre class="example">
|
|
<i>// Implicitly returns (), aka <a href="#stdlib-Void">Void</a></i>
|
|
func a() {}
|
|
|
|
<i>// Same as 'a'</i>
|
|
func a1() -> Void {}
|
|
|
|
<i>// Function pointers to a function expression.</i>
|
|
var a2 = func ()->() {}
|
|
var a3 = func () {}
|
|
var a4 = func {}
|
|
|
|
<i>// Really simple function</i>
|
|
func c(arg : Int) -> Int { return arg+4 }
|
|
|
|
<i>// Simple operators.</i>
|
|
func [infix_left=190] + (lhs : Int, rhs : Int) -> Int
|
|
func [infix_left=160] == (lhs : Int, rhs : Int) -> Bool
|
|
|
|
<i>// Curried function with multiple return values:</i>
|
|
func d(a : Int) (b : Int) -> (res1 : Int, res2 : Int) {
|
|
return (a,b)
|
|
}
|
|
|
|
<i>// A more realistic example on a trivial type.</i>
|
|
struct bankaccount {
|
|
amount : Int
|
|
|
|
static func bankaccount() -> bankaccount {
|
|
// Custom 'constructor' logic goes here.
|
|
}
|
|
func deposit(arg : Int) {
|
|
amount = amount + arg
|
|
}
|
|
|
|
static func someMetaTypeMethod() {}
|
|
}
|
|
|
|
<i>// Dot syntax on metatype.</i>
|
|
bankaccount.someMetaTypeMethod()
|
|
|
|
<i>// A function with selector-style signature.</i>
|
|
|
|
enum PersonOfInterest {
|
|
case ColonelMustard
|
|
case MissScarlet
|
|
}
|
|
enum Room {
|
|
case Conservatory
|
|
case Ballroom
|
|
}
|
|
enum Weapon {
|
|
case Candlestick
|
|
case LeadPipe
|
|
}
|
|
|
|
func accuseSuspect(suspect:PersonOfInterest)
|
|
inRoom(room:Room)
|
|
withWeapon(weapon:Weapon) {
|
|
println("It was \(suspect) in the \(room) with the \(weapon)")
|
|
}
|
|
|
|
<i>// Calling a selector-style function.</i>
|
|
accuseSuspect(.ColonelMustard, inRoom:.Ballroom, withWeapon:.LeadPipe)
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-typealias">typealias Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
We use the keyword "typealias" instead of "typedef" because it really is an
|
|
alias for an existing type, not a "definition" of a new type.
|
|
</div>
|
|
|
|
|
|
<pre class="grammar">
|
|
decl-typealias ::= typealias-head '=' <a href="#type">type</a>
|
|
<a name="typealias-head"></a>typealias-head ::= 'typealias' <a href="#identifier">identifier</a> <a href="#inheritance">inheritance</a>?
|
|
</pre>
|
|
|
|
<p>'typealias' makes a named alias of a type, like a typedef in C. From that
|
|
point on, the alias may be used in all situations the specified name is. If an <a href="#inheritance">inheritance</a> clause is provided, it specifies protocols to which the aliased type shall conform.</p>
|
|
|
|
<p>Here are some examples of type aliases:</p>
|
|
|
|
<pre class="example">
|
|
<i>// location is an alias for a tuple of ints.</i>
|
|
typealias location = (x : Int, y : Int)
|
|
|
|
<i>// pair_fn is a function that takes two ints and returns a tuple.</i>
|
|
typealias pair_fn = (Int) -> (Int) -> (first : Int, second : Int)
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-enum">enum Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-enum ::= 'enum' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> <a href="#generic-params">generic-params</a>? <a href="#inheritance">inheritance</a>? enum-body
|
|
enum-body ::= '{' decl* '}'
|
|
|
|
decl-enum-element ::= 'case' enum-case (',' enum-case)*
|
|
enum-case ::= <a href="#identifier">identifier</a> <a href="#type-tuple">type-tuple</a>? ('->' <a href="#type">type</a>)?
|
|
</pre>
|
|
|
|
<p>an enum declaration creates a <a href="#type-enum">enum type</a>.
|
|
Here are some examples of enum declarations:</p>
|
|
|
|
<pre class="example">
|
|
<i>// Declares three enums.</i>
|
|
enum DataSearchFlags {
|
|
case None
|
|
case Backward
|
|
case Anchored
|
|
}
|
|
|
|
<i>// Shorthand for the above.</i>
|
|
enum DataSearchFlags {
|
|
case None, Backward, Anchored
|
|
}
|
|
|
|
<i>// Declare discriminated union with enum decl.</i>
|
|
enum SomeInts {
|
|
case None
|
|
case One(Int)
|
|
case Two(Int, Int)
|
|
}
|
|
|
|
func f1(searchpolicy : DataSearchFlags) <i>// DataSearchFlags is a valid type name</i>
|
|
func test1() {
|
|
f1(DataSearchFlags.None) <i>// Use of constructor with qualified identifier</i>
|
|
f1(.None) <i>// Use of constructor with context sensitive type inference</i>
|
|
|
|
<i>// "None" has no type argument, so the constructor's type is "DataSearchFlags".</i>
|
|
var a : DataSearchFlags = .None
|
|
}
|
|
|
|
enum SomeMoreInts {
|
|
case None <i>// Doesn't conflict with previous "None".</i>
|
|
case One(Int)
|
|
case Two(Int, Int)
|
|
}
|
|
|
|
func f2(a : SomeMoreInts)
|
|
|
|
func test2() {
|
|
<i>// Constructors for enum element can be used in the obvious way.</i>
|
|
f2(.None)
|
|
f2(.One(4))
|
|
f2(.Two(1, 2))
|
|
|
|
<i>// Constructor for None has type "SomeMoreInts".</i>
|
|
var a : SomeMoreInts = SomeMoreInts.None
|
|
|
|
<i>// Constructor for One has type "(Int) -> SomeMoreInts".</i>
|
|
var b : (Int) -> SomeMoreInts = SomeMoreInts.One
|
|
|
|
<i>// Constructor for Two has type "(Int,Int) -> SomeMoreInts".</i>
|
|
var c : (Int,Int) -> SomeMoreInts = SomeMoreInts.Two
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-struct">struct Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-struct ::= 'struct' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> <a href="#generic-params">generic-params</a>? <a href="#inheritance">inheritance</a>? '{' decl-struct-body '}'
|
|
decl-struct-body ::= <a href="#decl">decl</a>*
|
|
</pre>
|
|
|
|
<p>A struct declares a simple value type that can contain data members and
|
|
have methods.</p>
|
|
|
|
<p>The body of a 'struct' is a list of decls. Non-property 'var' decls
|
|
declare members with storage in the struct. Other declarations act like
|
|
they would in an <a href="#decl-extension">extension</a> of the
|
|
struct type.</p>
|
|
|
|
<p>Here are a few simple examples:</p>
|
|
|
|
<pre class="example">
|
|
struct S1 {
|
|
var a : Int, b : Int
|
|
}
|
|
|
|
struct S2 {
|
|
var a : Int
|
|
func f() -> Int { return b }
|
|
var b : Int
|
|
}
|
|
</pre>
|
|
|
|
|
|
<p>Here are some more realistic examples of structs:</p>
|
|
|
|
<pre class="example">
|
|
struct Point { x : Int, y : Int }
|
|
struct Size { width : Int, height : Int }
|
|
struct Rect {
|
|
origin : Point,
|
|
size : Size
|
|
|
|
typealias CoordinateType = Int
|
|
|
|
func area() -> Int { return size.width*size.height }
|
|
}
|
|
|
|
func test4() {
|
|
var a : Point
|
|
var b = Point.Point(1, 2) // Silly but fine.
|
|
var c = Point(y = 1, x = 2) // Using metatype.
|
|
|
|
var x1 = Rect(a, Size(42, 123))
|
|
var x2 = Rect(size = Size(width = 42, height=123), origin = a)
|
|
|
|
var x1_area = x1.width*x1.height
|
|
var x1_area2 = x1.area()
|
|
}
|
|
</pre>
|
|
|
|
<div class="commentary">
|
|
Structs do not support inheritance due to undesirable ripple effects across
|
|
the design of the language. For example, method dispatch would arguably need
|
|
to become virtual, not static. The storage of the type would arguably need
|
|
to become indirected so that an array of T could be implemented sanely
|
|
(because we don't know if T is actually a T, or a subclass of T). We'd need
|
|
to store the "isa"/vtable in the struct so that virtual method dispatch
|
|
could be implemented, and this has additional storage costs. None of these
|
|
tradeoffs make sense for the intended use cases we have in mind for structs
|
|
(Ints, Floats, Points, Rects, UUIDs, IP addresses, C struct interop, etc,
|
|
etc). Said differently: we're trying to force a hard wall
|
|
between types that need indirect access by their nature and those types
|
|
that need direct access by their nature. The former are called classes. The
|
|
latter are called structs.
|
|
</div>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-class">class Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-class ::= 'class' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> <a href="#generic-params">generic-params</a>? <a href="#inheritance">inheritance</a>? '{' decl-class-body '}'
|
|
decl-class-body ::= <a href="#decl">decl</a>*
|
|
</pre>
|
|
|
|
<p>A class declares a reference type referring to an object which can contain
|
|
data members and have methods. Classes support single inheritance;
|
|
a parent class should be listed as the first type in the
|
|
inheritance list.</p>
|
|
|
|
<p>The body of a 'class' is a list of decls. Non-property 'var' decls
|
|
declare members with storage in the class. Non-static 'var' and 'func'
|
|
decls declare instance members; static 'var' and 'func' decls declare
|
|
members of the class itself. Both class and instance members can
|
|
be overridden by a subclass.</p>
|
|
<p>Type declarations inside a class act essentially the same way as type
|
|
declarations outside a class.</p>
|
|
|
|
<p>FIXME: For the moment, see classes.rst for more details on the
|
|
class system.</p>
|
|
<p>FIXME: Add a reference to the section on generics.</p>
|
|
|
|
<p>The only way to create a new instance of a class is with a
|
|
<a href="#expr-new">new expression</a>.
|
|
|
|
<p>Here is a simple example:</p>
|
|
|
|
<pre class="example">
|
|
class C1 {
|
|
var a : Int
|
|
var b : Int
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-protocol">Protocol Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-protocol ::= 'protocol' <a href="#attribute-list">attribute-list</a> <a href="#identifier">identifier</a> <a href="#inheritance">inheritance</a>? '{' protocol-member* '}'
|
|
</pre>
|
|
|
|
<p>A protocol declaration describes an abstract interface implemented by
|
|
another type. It consists of a set of declarations, which may be instance
|
|
methods or properties. A type <i>conforms</i> to a protocol if it
|
|
provides declarations that correspond to each of the declarations in
|
|
a protocol.</p>
|
|
|
|
<p>Here are some examples of protocols:</p>
|
|
|
|
<pre class="example">
|
|
protocol Document {
|
|
var title : String
|
|
}
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="protocol-member-func">'func' protocol elements</h4>
|
|
|
|
<pre class="grammar">
|
|
protocol-member ::= <a href="#decl-func">decl-func</a>
|
|
</pre>
|
|
|
|
<p>'func' members of a protocol define a value of function type that may be
|
|
accessed with dot syntax on a value of the protocol's type. The function
|
|
gets an implicit "self" argument of the protocol type and shall not
|
|
be static.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="protocol-member-var">'var' protocol elements</h4>
|
|
|
|
<pre class="grammar">
|
|
protocol-member ::= <a href="#decl-var">decl-var</a>
|
|
</pre>
|
|
|
|
<p>'var' members of a protocol define "property" values that may be accessed
|
|
with dot syntax on a value of the protocol's type. The actual
|
|
variables have no storage, and will instead by accessed by a getter
|
|
and setter. Thus, the variables shall have neither an initializer
|
|
nor a getter/setter clause.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="protocol-member-subscript">'subscript' protocol elements</h4>
|
|
|
|
<pre class="grammar">
|
|
protocol-member ::= <a href="#subscript-head">subscript-head</a>
|
|
</pre>
|
|
|
|
<p>'subscript' members of a protocol define subscripting operations
|
|
that may be accessed with the subscript operator ('[]') applied to a
|
|
value of the protocol's type. </p>
|
|
|
|
<div class="commentary">
|
|
TODO: There is currently no way to express a requirement for a
|
|
read-only or write-only subscript operation or variable. We may
|
|
end up doing this with some kind of 'const' or 'immutable'
|
|
attribute.
|
|
</div>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="protocol-member-typealias">'typealias' protocol elements (associated types)</h4>
|
|
|
|
<pre class="grammar">
|
|
protocol-member ::= <a href="#typealias-head">typealias-head</a>
|
|
</pre>
|
|
|
|
<p>'typealias' members of a protocol define associated types, which
|
|
are types used within the description of a protocol (typically in
|
|
the inputs and outputs of 'func' members) that vary from one
|
|
conforming type to another. When an associated type has an <a
|
|
href="#inheritance">inheritance</a> clause, any type meant to
|
|
satisfy the associated type requirement must conform to each of the
|
|
protocols specified within that inheritance clause.</p>
|
|
|
|
<pre class="example">
|
|
protocol Enumerable {
|
|
typename EnumeratorType : Enumerator
|
|
func getElements() -> EnumeratorType
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-subscript">subscript Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-subscript ::= subscript-head '{' <a href="#get-set">get-set<a/> '}'
|
|
<a id="subscript-head"></a>subscript-head ::= 'subscript' <a href="#attribute-list">attribute-list</a> <a href="#pattern-tuple">pattern-tuple</a> '->' <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p>A subscript declaration provides support for <a
|
|
href="#expr-subscript"> subscripting</a> an object of a particular
|
|
type via a getter and (optional) setter. Therefore, subscript
|
|
declarations can only appear within a type definition or
|
|
extension.</p>
|
|
|
|
<p> The <tt>pattern-tuple</tt> of a subscript declaration provides
|
|
the indices that will be used in the subscript expression, e.g., the
|
|
<tt>i</tt> in <tt>a[i]</tt>. This pattern must be fully-typed. The
|
|
<tt>type</tt> following the arrow provides the type of element being
|
|
accessed, which must be materializable. Subscript declarations can be
|
|
overloaded, so long as either the <tt>pattern-tuple</tt> or
|
|
<tt>type</tt> differs from other declarations.</p>
|
|
|
|
<p>The <tt>get-set</tt> clause specifies the getter and setter used
|
|
for subscripting. The getter is a function whose input is the type of
|
|
the <tt>pattern-tuple</tt> and whose result is the element type.
|
|
Similarly, the setter is a function whose result type is <tt>()</tt>
|
|
and whose input is the type of the <tt>pattern-tuple</tt> with a
|
|
parameter of the element type added to the end of the tuple; the name
|
|
of the parameter is the <tt>set-name</tt>, if provided, or
|
|
<tt>value</tt> otherwise.
|
|
|
|
<pre class="example">
|
|
<i>// Simple bit vector with storage for 64 boolean values</i>
|
|
struct BitVector64 {
|
|
bits : Int64
|
|
|
|
<i>// Allow subscripting with integer subscripts and a boolean result.</i>
|
|
subscript (bit : Int) -> Bool {
|
|
<i>// Getter tests the given bit</i>
|
|
get {
|
|
if (bits & (1 << bit)) != 0 {
|
|
return true
|
|
}
|
|
return false;
|
|
}
|
|
|
|
<i>// Setter sets the given bit to the provided value</i>
|
|
set {
|
|
var mask = 1 << bit
|
|
if value {
|
|
bits = bits | mask
|
|
} else {
|
|
bits = bits & ~mask
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
var vec : BitVector64
|
|
vec[2] = true
|
|
if vec[3] {
|
|
print("third bit is set\n");
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-constructor">constructor Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-constructor ::= 'init' <a href="#attribute-list">attribute-list</a> <a href="#generic-params">generic-params</a>? constructor-signature <a href="#brace-item-list">brace-item-list</a>
|
|
|
|
constructor-signature ::= <a href="#pattern-tuple">pattern-tuple</a>
|
|
constructor-signature ::= <a href="#identifier">identifier-or-any</a> <a href="#selector-tuple">selector-tuple</a>
|
|
</pre>
|
|
|
|
<p>'constructor' declares a constructor for a class, struct, or enum. Such
|
|
a declaration is used whenever an object is constructed. Specifically,
|
|
for classes, it is used when a new expression is written, and for structs
|
|
and enums, it is used for function application when the "function"
|
|
is a metatype.</p>
|
|
|
|
<p>FIXME: We haven't decided the precise rules for when constructors are
|
|
implicitly declared. Default construction doesn't work right for structs
|
|
or enums. We haven't decided what the restrictions are if a member
|
|
isn't default-constructible.</p>
|
|
|
|
<p>A simple example:</p>
|
|
|
|
<pre class="example">
|
|
struct X {
|
|
var member : Int
|
|
init(x : Int) {
|
|
member = x
|
|
}
|
|
}
|
|
var a = X(10)
|
|
</pre>
|
|
|
|
<p>If a class is derived from a superclass, it must explicitly invoke a
|
|
superclass constructor using the <tt>super.init</tt> syntax.
|
|
<tt>super.init</tt> may only be used in a subclass constructor;
|
|
it is not valid in a struct, enum, or root class constructor. Additionally,
|
|
<tt>super.init</tt> may only be referenced exactly once per derived
|
|
constructor. An example:</p>
|
|
|
|
<pre class="example">
|
|
class View {
|
|
var bounds : Rect
|
|
init(bounds:Rect) {
|
|
self.bounds = bounds
|
|
}
|
|
}
|
|
|
|
class Button : View {
|
|
var onClick : () -> ()
|
|
init(bounds:Rect, onClick:() -> ()) {
|
|
super.init(bounds)
|
|
self.onClick = onClick
|
|
}
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="decl-destructor">destructor Declarations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
decl-constructor ::= 'destructor' <a href="#attribute-list">attribute-list</a> <a href="#brace-item-list">brace-item-list</a>
|
|
</pre>
|
|
|
|
<p>'destructor' declares a destructor for a class. This function is called
|
|
when there are no longer any references to a class object, just before it
|
|
is destroyed. Note that destructors can only be declared for classes,
|
|
and cannot be declared in extensions. Subclass destructors implicitly
|
|
invoke their superclass destructors after executing.</p>
|
|
|
|
<p>FIXME: We haven't really decided the precise rules here, but it's probably
|
|
a fatal error to either throw an exception or stash a reference to 'self'
|
|
in a destructor. Not sure what happens when we cause the reference count
|
|
of another object to reach zero inside a destructor. We might eventually
|
|
allow destructors in extensions once we have ivars in extensions.</p>
|
|
|
|
<p>A simple example:</p>
|
|
|
|
<pre class="example">
|
|
class X {
|
|
var fd : Int
|
|
destructor {
|
|
close(fd)
|
|
}
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="attribute-list">Attribute Lists</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
attribute-list ::= /*empty*/
|
|
attribute-list ::= attribute-list-clause attribute-list
|
|
attribute-list-clause ::= '[' ']'
|
|
attribute-list-clause ::= '[' attribute (',' attribute)* ']'
|
|
|
|
attribute ::= attribute-infix
|
|
attribute ::= attribute-resilience
|
|
attribute ::= attribute-byref
|
|
attribute ::= attribute-auto_closure
|
|
attribute ::= attribute-noreturn
|
|
</pre>
|
|
|
|
<p>An attribute list is written as a sequence of clauses delimited
|
|
by square brackets, each of which contains a (possibly empty)
|
|
comma-separated list of attributes. Neither the ordering of
|
|
attributes nor the grouping of attributes into separate clauses
|
|
has any semantic effect. Attributes may not be repeated within a
|
|
list.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="attribute-infix">Infix Attributes</h4>
|
|
|
|
<pre class="grammar">
|
|
attribute-infix ::= 'infix_left' '=' <a href="#integer_literal">integer_literal</a>
|
|
attribute-infix ::= 'infix_right' '=' <a href="#integer_literal">integer_literal</a>
|
|
attribute-infix ::= 'infix '=' <a href="#integer_literal">integer_literal</a>
|
|
</pre>
|
|
|
|
<p>The infix attributes may only be applied to the declaration of a
|
|
function of binary operator type whose name is an
|
|
<a href="#operator"><tt>operator</tt></a>. The name indicates the
|
|
associativity of the operator, either left associative, right associative, or
|
|
non-associative.</p>
|
|
|
|
<p>FIXME: Implement these restrictions.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="attribute-resilence">Resilience Attribute</h4>
|
|
|
|
<pre class="grammar">
|
|
attribute-resilience ::= 'resilient'
|
|
attribute-resilience ::= 'fragile'
|
|
attribute-resilience ::= 'born_fragile'
|
|
</pre>
|
|
|
|
<p>See the resilience design.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="attribute-byref">By-Reference Attribute</h4>
|
|
|
|
<pre class="grammar">
|
|
attribute-byref ::= 'byref'
|
|
</pre>
|
|
|
|
<p><tt>byref</tt> is only valid in a <tt>type-annotation</tt> that
|
|
appears within either a <a href="#pattern"><tt>pattern</tt></a> of
|
|
a <tt>function-signature</tt> or the input type of a function
|
|
type.
|
|
</p>
|
|
|
|
<p><tt>byref</tt> indicates that the argument will be passed "by
|
|
reference": the bound variable will be an l-value.</p>
|
|
|
|
<p>The type being annotated must be <a href="#materializable">materializable</a>.
|
|
The type after annotation is never materializable.</tt>
|
|
|
|
<p>FIXME: we probably need a const-like variant, which permits
|
|
r-values (and avoids writeback when the l-value is not physical).
|
|
We may also need some way of representing <q>this will be
|
|
consumed by the nth curry</q>.
|
|
</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="attribute-auto_closure">auto_closure Attribute</h4>
|
|
|
|
<pre class="grammar">
|
|
attribute-auto_closure ::= 'auto_closure'
|
|
</pre>
|
|
|
|
<p>The <tt>auto_closure</tt> attribute modifies a <a
|
|
href="#type-function">function type</a>, changing the behavior of any
|
|
assignment into (or initialization of) a value with the function type.
|
|
Instead of requiring that the rvalue and lvalue have the same function type,
|
|
an "auto closing" function type requires its initializer expression to have
|
|
the same type as the function's result type, and it implicitly binds a
|
|
closure over this expression. This is typically useful for function arguments
|
|
that want to capture computation that can be run lazily.</p>
|
|
|
|
<p><tt>auto_closure</tt> is only valid in a <tt>type-annotation</tt> of a
|
|
syntactic function type that is defined to take a syntactic empty tuple.
|
|
</p>
|
|
|
|
<pre class="example">
|
|
<i>// An auto closure value. This captures an implicit closure over the</i>
|
|
<i>// specified expression, instead of the expression itself.</i>
|
|
var a : [auto_closure] () -> Int = 4
|
|
|
|
<i>// Definition of an 'assert' function. Assertions and logging routines</i>
|
|
<i>// often want to conditionally evaluate their argument.</i>
|
|
func assert(condition : [auto_closure] () -> Bool)
|
|
|
|
<i>// Definition of the || operator - it captures its right hand side as</i>
|
|
<i>// an autoclosure so it can short-circuit evaluate it.</i>
|
|
func [infix_left=110] || (lhs: Bool, rhs: [auto_closure] ()->Bool) -> Bool
|
|
|
|
<i>// Example uses of these functions:</i>
|
|
assert(i < j)
|
|
if (a == 0 || b == 42) { ... }
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h4 id="attribute-noreturn">No Return Attribute</h4>
|
|
|
|
<pre class="grammar">
|
|
attribute-noreturn ::= 'noreturn'
|
|
</pre>
|
|
|
|
<p>Attribute <tt>noreturn</tt> is only valid in the attribute list of a
|
|
function declaration or in the attribute list of a <tt>type-annotation</tt>
|
|
that describes a syntactic function type.
|
|
</p>
|
|
|
|
<p><tt>noreturn</tt> indicates to the compiler that the function will not
|
|
return to the caller. This attribute should be used to suppress the
|
|
uninitialized variable, missing return warnings and errors. The compiler is
|
|
also allowed to more aggressively optimize the code in presence of this
|
|
attribute.
|
|
</p>
|
|
|
|
<p>If a function with no a <tt>noreturn</tt> attribute contains a
|
|
<tt>return</tt> statement, an error will be raised.
|
|
</p>
|
|
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="type">Types</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
type ::= <a href="#type-function">type-function</a>
|
|
type ::= <a href="#type-array">type-array</a>
|
|
|
|
type-simple ::= <a href="#type-identifier">type-identifier</a>
|
|
type-simple ::= <a href="#type-tuple">type-tuple</a>
|
|
type-simple ::= <a href="#type-composition">type-composition</a>
|
|
type-simple ::= <a href="#type-metatype">type-metatype</a>
|
|
type-simple ::= <a href="#type-optional">type-optional</a>
|
|
|
|
type-annotation ::= <a href="#attribute-list">attribute-list</a> type
|
|
</pre>
|
|
|
|
<p>Swift has a small collection of core datatypes that are built into the
|
|
compiler. Most user-facing datatypes are defined by the
|
|
<a href="#stdlib">standard library</a> or declared as a user defined
|
|
types.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3>Metatypes</h3>
|
|
|
|
<p id="metatype">Each type has a corresponding <i>metatype</i>, with the same
|
|
name as the type, that is injected into the standard name lookup scope when
|
|
a type is <a href="#decl">declared</a>. This allows access to '<a
|
|
href="#decl-func">static functions</a>' through dot syntax. For example:</p>
|
|
|
|
<pre class="example">
|
|
// Declares a type 'foo' as well as its metatype.
|
|
struct foo {
|
|
static func bar() {}
|
|
}
|
|
|
|
// Declares x to be of type foo. A reference to a name in type context
|
|
// refers to the type itself.
|
|
var x : foo
|
|
|
|
// Accesses a static function on the foo metatype. In a value context, the
|
|
// name of its type refers to its metatype.
|
|
foo.bar()
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3 id="fully_typed_types">Fully-Typed Types</h3>
|
|
|
|
<p>A type may be <i>fully-typed</i>. A type is fully-typed <i>unless</i> one
|
|
of the following conditions hold:</p>
|
|
|
|
<ol>
|
|
<li>It is a function type whose result or input type is not
|
|
fully-typed.</li>
|
|
<li>It is a tuple type with an element that is not
|
|
fully-typed. A tuple element is fully-typed unless it has no
|
|
explicit type (which is permitted for defaultable elements) or its
|
|
explicit type is not fully-typed. In other words, a type is
|
|
fully-typed unless it syntactically contains a tuple element with
|
|
no explicit type annotation.</li>
|
|
</ol>
|
|
|
|
<p>A type being 'fully-typed' informally means that the type is specified
|
|
directly from its type annotation without needing contextual or other
|
|
information to resolve its type.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3>Materializable Types</h3>
|
|
|
|
<p id="materializable">A type may be <i>materializable</i>. A type
|
|
is materializable unless it is 1) annotated with
|
|
a <a href="#attribute-byref"><tt>byref</tt></a> attribute or 2) a
|
|
tuple with a non-materializable element type. In general, variables
|
|
must have materializable type.</p>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="type-identifier">Named Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
type-identifier ::= type-identifier-component ('.' type-identifier-component)*
|
|
type-identifier-component ::= <a href="#identifier">identifier</a> <a href="#generic-args">generic-args</a>?
|
|
</pre>
|
|
|
|
<p>Named types may be used simply by using their name. Named types are
|
|
introduced by <a href="#decl-typealias">typealias</a> declarations or
|
|
through type declarations that expand to one.</p>
|
|
|
|
<pre class="example">
|
|
typealias location = (x : Int, y : Int)
|
|
var x : location <i>// use of a named type.</i>
|
|
</pre>
|
|
|
|
<p>Type names may use dot syntax to refer to names types declared in other
|
|
modules or types nested within other types.</p>
|
|
|
|
<pre class="example">
|
|
<i>// Direct reference to a member of another module.</i>
|
|
var x : swift.Int
|
|
</pre>
|
|
|
|
<p>Each component of a named type may be followed by a list of generic
|
|
parameters for that component enclosed in angle brackets <tt><></tt>.
|
|
|
|
<pre class="example">
|
|
<i>// A generic class definition.</i>
|
|
class Dict<K, V> { }
|
|
|
|
<i>// A variable of a generic instance type.</i>
|
|
var map : Dict<String, Int>
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="type-tuple">Tuple Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
Tuples are everywhere in Swift: even the argument list of a function is a
|
|
tuple of those arguments.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
type-tuple ::= '(' type-tuple-body? ')'
|
|
type-tuple-body ::= type-tuple-element (',' type-tuple-element)* '...'?
|
|
type-tuple-element ::= identifier ':' <a href="#type">type-annotation</a>
|
|
type-tuple-element ::= <a href="#type">type-annotation</a>
|
|
</pre>
|
|
|
|
<p>Syntactically, tuple types are simply a (possibly empty) list of
|
|
elements enclosed in parentheses. A tuple type with a single, anonymous
|
|
element is exactly that type: the parentheses are treated as
|
|
grouping parentheses.</p>
|
|
|
|
<p>Tuples are the low-level form of data aggregation in Swift, and are used as
|
|
the building block of <a href="#type-function">function</a> argument lists,
|
|
multiple return values, <a href="#decl-enum">enum</a> bodies, etc. Because
|
|
tuples are widely accessible and available everywhere in the language,
|
|
aggregate data access and transformation is uniform and powerful.</p>
|
|
|
|
<p>Each element of a tuple contains an optional name followed by a type.</p>
|
|
|
|
<p>If the tuple body ends with '...', the tuple is a varargs tuple. The type
|
|
of the last element is changed from T to T[], and there are special rules
|
|
for converting an expression to varargs tuple type.</p>
|
|
|
|
<pre class="example">
|
|
<i>// Variable definitions.</i>
|
|
var a : ()
|
|
var b : (Int, Int)
|
|
var c : (x : (), y : Int)
|
|
|
|
<i>// Tuple type inferred from an initializers:</i>
|
|
var m = () <i>// Type = ()</i>
|
|
var n = (x = 1, y = 2) <i>// Type = (x : Int, y : Int)</i>
|
|
var o = (1, 2, 3) <i>// Type = (Int, Int, Int)</i>
|
|
|
|
<i>// Function argument and result is a tuple type.</i>
|
|
func foo(x : Int, y : Int) -> (val : Int, err : Int)
|
|
|
|
<i>// enum and struct declarations with tuple values.</i>
|
|
struct S {
|
|
var (a : Int, b : Int)
|
|
}
|
|
enum Vertex {
|
|
case Point2(x : Int, y : Int)
|
|
case Point3(x : Int, y : Int, z : Int)
|
|
case Point4(w : Int, x : Int, y : Int, z : Int)
|
|
}
|
|
</pre>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="type-function">Function Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
type-function ::= <a href="#type">type-tuple</a> '->' <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p>Function types have a single input and single result type, separated by
|
|
an arrow. Because each of the types is allowed to be a tuple, we trivially
|
|
support multiple arguments and multiple results. "Function" types are
|
|
more properly known as a "closure" type, because they can embody any
|
|
context captured when the function value was formed.</p>
|
|
|
|
<p>The result type of a function type must
|
|
be <a href="#materializable">materializable</a>. The argument type of a
|
|
function is always required to be parenthesized (a tuple). The behavior
|
|
of function types may be modified with the <a
|
|
href="#attribute-auto_closure"><tt>auto_closure</tt> attribute</a>.</p>
|
|
|
|
<p>Because of the grammar structure, a nested function type like
|
|
"(a) -> (b) -> c" is parsed as "(a) -> ((b) -> c)". This means
|
|
that if
|
|
you declare this that you can pass it one argument to get a function that
|
|
"takes b and returns c" or you can pass two arguments to "get a c". This
|
|
is known as <a href="http://en.wikipedia.org/wiki/Currying">currying</a>.
|
|
For example:
|
|
</p>
|
|
|
|
<pre class="example">
|
|
<i>// A simple function that takes a tuple and returns Int:</i>
|
|
var a : (a : Int, b : Int) -> Int
|
|
|
|
<i>// A simple function that returns multiple values:</i>
|
|
var a : (a : Int, b : Int) -> (val: Int, err: Int)
|
|
|
|
<i>// Declare a function that returns a function:</i>
|
|
var x : (Int) -> (Int) -> Int
|
|
|
|
<i>// y has type (Int) -> Int</i>
|
|
var y = x(1)
|
|
|
|
<i>// z1 and z2 both has type Int, and both have the same value (assuming
|
|
// the function had no side effects).</i>
|
|
var z1 = x(1)(2)
|
|
var z2 = y(2)
|
|
|
|
<i>// An auto closure value. This captures an implicit closure over the</i>
|
|
<i>// specified expression, instead of the expression itself.</i>
|
|
var a : [auto_closure] () -> Int = 4
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="type-enum">Enum Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
'enum' types are known as <a
|
|
href="http://en.wikipedia.org/wiki/Algebraic_data_type">algebraic data
|
|
types</a> (ADTs) by the broader programming language community.
|
|
We name them 'enum' after C enums, because ADTs
|
|
fulfill many of the same roles as enums in the C tradition.
|
|
</div>
|
|
|
|
<p>an enum type is a simple discriminated union: the runtime representation
|
|
of a value of enum type only has one of the specified elements at a time.</p>
|
|
|
|
<p>All of the element types of an enum type must
|
|
be <a href="#materializable">materializable</a>.</p>
|
|
|
|
<p>an enum type is defined by a <a href="#decl-enum">enum decl</a>.
|
|
|
|
<p>Values of enum type may not be default initialized unless the user
|
|
provides a no-argument constructor.</p>
|
|
|
|
<p>The enum metatype has a member corresponding to each declared element.
|
|
For elements with a declared type, this member is a function which can
|
|
construct an enum containing that element. For elements without a
|
|
declared type, the member is simply an enum value for that element. A
|
|
enum value has no accessible members except those explicitly defined
|
|
by the user.</p>
|
|
|
|
<p>A reference to a member of the enum metatype can be shortened using <a
|
|
href="#expr-delayed-identifier">delayed identifier resolution</a>
|
|
with <a href="#typecheck_context">context sensitive type inference</a>.
|
|
</p>
|
|
|
|
<p>The enum's value can be tested and accessed by pattern-matching the enum
|
|
against a <a href="#pattern-enum-element">enum element pattern</a>.
|
|
|
|
<p>TODO: Should attributes be allowed on enum elements?
|
|
TODO: Eventually, with generics we'll have equality and inequality operators.
|
|
Enum decls should be able to implicitly define these for their types.
|
|
TODO: Need pattern matching and element extraction.
|
|
</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="type-array">Array Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
Note that array types are parsed inside-out, with the first
|
|
bounds clause being the outermost one. This little oddity is required
|
|
for the bounds of nested arrays to correspond in sequence to subscript
|
|
indexes. That is, given an array "x : Int[5][7][11][13]" and a
|
|
chained subscript expression of the form "x[i][j][k][l]", we really
|
|
want "i" to be bounded by 5, "j" by 7, and so on. This is probably
|
|
the only case where C's rule of "declaration follows use" really makes
|
|
sense. There's precedent for this in many languages, including Java and C#.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
type-array ::= <a href="#type">type-simple</a>
|
|
type-array ::= <a href="#type">type-array</a> '[' ']'
|
|
type-array ::= <a href="#type">type-array</a> '[' <a href="#expr">expr</a> ']'
|
|
</pre>
|
|
|
|
<p>Array types include a base type and an optional size. Array types indicate
|
|
a linear sequence of elements stored consequtively memory. Array elements may
|
|
be efficiently indexed in constant time. All array indexes are bounds checked
|
|
and out of bound accesses are diagnosed with either a compile time or
|
|
runtime failure (TODO: runtime failure mode not specified).</p>
|
|
|
|
<p>While they look syntactically very similar, an array type with a size has
|
|
very different semantics than an array without. In the former case, the type
|
|
indicates a declaration of actual storage space. In the later case, the type
|
|
indicates a <em>reference</em> to storage space allocated elsewhere of
|
|
runtime-specified size.
|
|
</p>
|
|
|
|
<p>FIXME: We should separate out "Arrays" from "Slices". Arrays should always
|
|
require a size and is by-value, a slice is a by-ref and never have a
|
|
(statically specified) size.</p>
|
|
|
|
<p>For an array with a size, the size must be more than zero (no
|
|
indices would be valid). For now, the array size must be a literal integer.
|
|
TODO: Define a notion like C's integer-constant-expression for how constant
|
|
folding works.</p>
|
|
|
|
<p>The element type of an array type must
|
|
be <a href="#materializable">materializable</a>.</p>
|
|
|
|
<p>FIXME: Int[][] not valid because the element type isn't sized. We need
|
|
some constraint to reject this, or do we?</p>
|
|
|
|
<p>Some example array types:</p>
|
|
|
|
<pre class="example">
|
|
<i>// A simple array declaration:</i>
|
|
var a : Int[4]
|
|
|
|
<i>// A reference to another array:</i>
|
|
var b : Int[] = a
|
|
|
|
<i>// Declare a two dimensional array:</i>
|
|
var c : Int[4][4]
|
|
|
|
<i>// Declare a reference to another array, two dimensional:</i>
|
|
var d : Int[4][]
|
|
|
|
<i>// Declare an array of function pointers:</i>
|
|
var array_fn_ptrs : (: (Int) -> Int)[42]
|
|
var g = array_fn_ptrs[12](4)
|
|
|
|
<i>// Without parens, this is a function that returns a fixed size array:</i>
|
|
var fn_returning_array : (Int) -> Int[42]
|
|
var h : Int[42] = fn_returning_array(4)
|
|
|
|
<i>// You can even have arrays of tuples and other things, these work right
|
|
// through composition:</i>
|
|
var array_of_tuples : (a : Int, b : Int)[42]
|
|
var tuple_of_arrays : (a : Int[42], b : Int[42])
|
|
|
|
array_of_tuples[12].a = array_of_tuples[13].b
|
|
tuple_of_arrays.a[12] = array_of_tuples.b[13]
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3 id="type-metatype">Metatype Types</h3>
|
|
<pre class="grammar">
|
|
type-metatype ::= type-simple '.' 'metatype'
|
|
</pre>
|
|
|
|
<p>Every type has an associated metatype. A value of the metatype
|
|
type is a reference to a global object which describes the type.
|
|
Most metatype types are singleton and therefore require no
|
|
storage, but metatypes associated with <a href="#decl-class">class
|
|
types</a> follow the same subtyping rules as their associated
|
|
class types and therefore are not singleton.</p>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3 id="type-optional">Optional Types</h3>
|
|
<div class="commentary">
|
|
Similar constructs exist in Haskell (<a
|
|
href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Maybe.html">Maybe</a>),
|
|
the Boost library (<a
|
|
href="http://www.boost.org/doc/libs/1_54_0/libs/optional/doc/html/index.html">Optional</a>),
|
|
and C++14 (<a href="http://en.cppreference.com/w/cpp/utility/optional">optional</a>).
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
type-optional ::= type-simple '?'
|
|
</pre>
|
|
|
|
<p>An optional type is syntactic sugar for the library type
|
|
Optional<T>. This is a <a href="#decl-enum">enum</a> with two
|
|
cases: None and Some, used to represent a value that may or may not be
|
|
present.</p>
|
|
|
|
<p>FIXME: The current implementation of Optional is a hack and None and
|
|
Some are currently both globals rather than members.</p>
|
|
|
|
<p>Optional types are different from other enums in that any value
|
|
<code>x</code> is implicitly convertible to Some(<code>x</code>). This is
|
|
part of the language, not the library; therefore, the libary type Optional
|
|
is required to have a constructor that accepts a single argument.</p>
|
|
|
|
<p>Since optional types are part of the
|
|
<a href="#type-simple">type-simple</a> grammar, it is not possible to write
|
|
<code>T[]?</code> for an optional array. Use <code>(T[]?)</code> instead.
|
|
</p>
|
|
|
|
<p>Some example optional types:</p>
|
|
|
|
<pre class="example">
|
|
<i>// A simple optional declaration:</i>
|
|
var a : Int? // equivalent to Optional<Int>
|
|
|
|
<i>// An empty optional:</i>
|
|
var b : Int? = .None
|
|
|
|
<i>// Declare an array of optionals:</i>
|
|
var c : Int?[] = new Int?[4]
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3 id="type-composition">Protocol Composition Types</h3>
|
|
<pre class="grammar">
|
|
type-composition ::= 'protocol' '<' type-composition-list? '>'
|
|
|
|
type-composition-list ::= <a href="#type-identifier">type-identifier</a> (',' <a href="#type-identifier">type-identifier</a>)*
|
|
</pre>
|
|
|
|
<p>A protocol composition type composes together a number of
|
|
protocols to describe a type that meets the requirements of each of
|
|
those protocols. A protocol composition type <code>protocol<A,
|
|
B></code> is similar to an explicitly-defined protocol that
|
|
inherits both <code>A</code> and <code>B</code></p>
|
|
|
|
<pre class="example">
|
|
protocol C : A, B { }
|
|
</pre>
|
|
|
|
<p>but without the need to introduce a new name.</p>
|
|
|
|
<div class="commentary">
|
|
If we drop implicit conformance to protocols, protocol composition
|
|
types become much more important, because they allow you to give a
|
|
name to a composition without requiring types to explicitly
|
|
conform to that name.
|
|
</div>
|
|
|
|
<p>Each of the types named in the
|
|
<code>type-composition-list</code> shall refer to either a protocol
|
|
or to a protocol composition. The list may be empty, in which case
|
|
every type conforms to the empty protocol composition. This is how
|
|
the <code>Any</code> type is defined in the standard library.</p>
|
|
|
|
<pre class="example">
|
|
<i>// A value that represents any type</i>
|
|
var any : protocol<> = 17
|
|
|
|
<i>// A value that conforms to both the Document and Enumerator protocols</i>
|
|
var doc : protocol<Document,Enumerator>
|
|
doc.isEmpty() <i>// uses Enumerator.isEmpty()</i>
|
|
doc.title = "Hello" <i>// uses Document.title</i>
|
|
</pre>
|
|
|
|
<!-- _____________________________________________________________________ -->
|
|
<h3 id="inheritance">Type Inheritance</h3>
|
|
<pre class="grammar">
|
|
inheritance ::= ':' <a href="#type-identifier">type-identifier</a> (',' <a href="#type-identifier">type-identifier</a>)*
|
|
</pre>
|
|
|
|
<p>A named type (e.g., a class, struct, enum, or protocol) can
|
|
"inherit" some set of protocols, which implies that any object of
|
|
that type conforms to each of those protocols. When a protocol
|
|
inherits other protocols, the set of requirements from all of those
|
|
protocols is effectivel aggregated into the protocol, and a type that
|
|
conforms to the current protocol shall conform to each of the
|
|
protocols that it inherits.</p>
|
|
|
|
<p>When a non-protocol type inherits a protocol, it is specifying
|
|
explicitly that it conforms to that protocol. The program is
|
|
ill-formed if the type does not conform to the protocol.</p>
|
|
|
|
<pre class="example">
|
|
protocol VersionedDocument : Document { <i>// every VersionedDocument is a Document</i>
|
|
func bumpVersion()
|
|
}
|
|
|
|
func print(doc : Document) { <i>/* ... */</i> }
|
|
|
|
var myDocument : VersionedDocument;
|
|
print(myDocument) <i>// okay: a VersionedDocument is a Document</i>
|
|
|
|
class StoredHTML : VersionedDocument { <i>// okay: StoredHTML conforms to VersionedDocument</i>
|
|
var Title : String
|
|
func bumpVersion()
|
|
}
|
|
</pre>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="pattern">Patterns</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<div class="commentary">
|
|
The pattern grammar mirrors the expression grammar, or to be more
|
|
specific, the grammar of literals. This is because the conceptual
|
|
algorithm for matching a value against a pattern is to try to find
|
|
an assignment of values to variables which makes the pattern equal
|
|
the value. So every expression form which can be used to build a
|
|
value directly should generally have a corresponding pattern form.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
pattern-atom ::= <a href="#pattern-var">pattern-var</a>
|
|
pattern-atom ::= <a href="#pattern-any">pattern-any</a>
|
|
pattern-atom ::= <a href="#pattern-tuple">pattern-tuple</a>
|
|
pattern-atom ::= <a href="#pattern-is">pattern-is</a>
|
|
pattern-atom ::= <a href="#pattern-enum-element">pattern-enum-element</a>
|
|
pattern-atom ::= <a href="#pattern-expr">expr</a>
|
|
|
|
pattern ::= pattern-atom
|
|
pattern ::= <a href="#pattern-typed">pattern-typed</a>
|
|
</pre>
|
|
|
|
<p>A pattern represents the structure of a composite value.
|
|
Parts of a value can be extracted and bound to variables or compared
|
|
against other values by <i>pattern matching</i>. Among other places,
|
|
pattern matching occurs on the left-hand side of
|
|
<a href="#decl-var"><tt>var</tt> bindings</a>, in the arguments of
|
|
<a href="#decl-func"><tt>func</tt> declarations</a>, and in the
|
|
<tt>case</tt> labels of
|
|
<a href="#stmt-switch"><tt>switch</tt> statements</a>.
|
|
Some examples:</p>
|
|
|
|
<pre class="example">
|
|
var point = (1, 0, 0)
|
|
|
|
<i>// Extract the elements of the "point" tuple and bind them to
|
|
// variables x, y, and z.</i>
|
|
var (x, y, z) = point
|
|
println("x=\(x) y=\(y) z=\(z)")
|
|
|
|
<i>// Dispatch on the elements of a tuple in a "switch" statement.</i>
|
|
switch point {
|
|
case (0, 0, 0):
|
|
println("origin")
|
|
<i>// The pattern "_" matches any value.</i>
|
|
case (_, 0, 0):
|
|
println("on the x axis")
|
|
case (0, _, 0):
|
|
println("on the y axis")
|
|
case (0, 0, _):
|
|
println("on the z axis")
|
|
case (var x, var y, var z):
|
|
println("x=\(x) y=\(y) z=\(z)")
|
|
}
|
|
</pre>
|
|
|
|
<p>A pattern may be "irrefutable", meaning informally that it
|
|
matches all values of its type. Patterns in declarations, such as
|
|
<a href="#decl-var"><tt>var</tt></a> and
|
|
<a href="#decl-func"><tt>func</tt></a>, are required to be irrefutable.
|
|
Patterns in the <tt>case</tt> labels of
|
|
<a href="#stmt-switch"><tt>switch</tt> statements</a>, however,
|
|
are not.</p>
|
|
|
|
<p>The basic pattern grammar is a literal "atom" followed by an
|
|
optional type annotation. Type annotations are useful for
|
|
documentation, as well as for coercing a matched expression to a
|
|
particular kind. They are also required when patterns are used in
|
|
a <a href="#func-signature">function signature</a>. Type annotations
|
|
are currently not allowed in switch statements.</p>
|
|
|
|
<p>A pattern has a type. A pattern may be "fully-typed", meaning
|
|
informally that its type is fully determined by the type
|
|
annotations it contains. Some patterns may also derive a type
|
|
from their context, be it an enclosing pattern or the way it is
|
|
used; this set of situations is not yet fully determined.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-typed">Typed Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-typed ::= pattern-atom ':' <a href="#type">type-annotation</a>
|
|
</pre>
|
|
|
|
<p>A type annotation constrains a pattern to have a specific type.
|
|
An annotated pattern is fully-typed if its annotation type is
|
|
fully-typed. It is irrefutable if and only if its subpattern is
|
|
irrefutable.</p>
|
|
|
|
<p>Type annotations are currently not allowed in the <tt>case</tt> labels
|
|
of <tt>switch</tt> statements; case patterns always get their type from
|
|
the subject of the switch.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-any">Any Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-any ::= '_'
|
|
</pre>
|
|
|
|
<p>The symbol <tt>_</tt> in a pattern matches and ignores any value. It
|
|
is irrefutable.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-var">'var' Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-var ::= 'var' pattern
|
|
</pre>
|
|
|
|
<p>The keyword <tt>var</tt> within a pattern introduces variable bindings.
|
|
Any identifiers within the subpattern bind new named variables to their
|
|
matching values.<p>
|
|
|
|
<pre class="example">
|
|
var point = (0, 0, 0)
|
|
switch point {
|
|
<i>// Bind x, y, z to the elements of point.</i>
|
|
case (var x, var y, var z):
|
|
println("x=\(x) y=\(y) z=\(z)")
|
|
}
|
|
|
|
switch point {
|
|
<i>// Same. 'var' distributes to the identifiers in its subpattern.</i>
|
|
case var (x, y, z):
|
|
println("x=\(x) y=\(y) z=\(z)")
|
|
}
|
|
</pre>
|
|
|
|
<p>Outside of a <tt>var</tt> pattern, an identifier behaves as an
|
|
<a href="#pattern-expr">expression pattern</a> referencing an existing
|
|
definition.</p>
|
|
|
|
<pre class="example">
|
|
var zero = 0
|
|
switch point {
|
|
<i>// x and z are bound as new variables.
|
|
// zero is a reference to the existing 'zero' variable.</i>
|
|
case (var x, zero, var z):
|
|
println("point off the y axis: x=\(x) z=\(z)")
|
|
default:
|
|
println("on the y axis")
|
|
}
|
|
</pre>
|
|
|
|
<p>The left-hand pattern of a <a href="#decl-var"><tt>var</tt> declaration</a>
|
|
and the argument pattern of a <a href="#decl-func"><tt>func</tt> declaration</a>
|
|
are implicitly inside a <tt>var</tt> pattern; identifiers in their patterns
|
|
always bind variables. Variable bindings are irrefutable.</p>
|
|
|
|
<p>The type of a bound variable must be
|
|
<a href="#materializable">materializable</a> unless it appears in a
|
|
<a href="#function-signature">function-signature</a> and is directly of
|
|
a <a href="attribute-byref"><tt>byref</tt></a>-annotated type.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-tuple">Tuple Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-tuple ::= '(' pattern-tuple-body? ')'
|
|
pattern-tuple-body ::= pattern-tuple-element (',' pattern-tuple-body)* '...'?
|
|
pattern-tuple-element ::= pattern
|
|
pattern-tuple-element ::= pattern '=' <a href="#expr">expr</a>
|
|
</pre>
|
|
|
|
<p>A tuple pattern is a list of zero or more patterns. Within a
|
|
function signature, patterns may also be given a default-value
|
|
expression.</p>
|
|
|
|
<p>A tuple pattern is irrefutable if all its sub-patterns are
|
|
irrefutable.</p>
|
|
|
|
<p>A tuple pattern is fully-typed if all its sub-patterns are
|
|
fully-typed, in which case its type is the corresponding tuple
|
|
type, where each <tt>type-tuple-element</tt> has the type,
|
|
label, and default value of the corresponding <tt>pattern-tuple-element</tt>.
|
|
A <tt>pattern-tuple-element</tt> has a label if it is a named
|
|
pattern or a type annotation of a named pattern.</p>
|
|
|
|
<p>A tuple pattern whose body ends in <tt>'...'</tt> is a varargs tuple.
|
|
The last element of such a tuple must be a typed pattern, and the type
|
|
of that pattern is changed from <tt>T</tt> to <tt>T[]</tt>. The
|
|
corresponding tuple type for a varargs tuple is a varargs tuple type.</p>
|
|
|
|
<p>As a special case, a tuple pattern with one element that has no
|
|
label, has no default value, and is not varargs is treated as a
|
|
grouping parenthesis: it has the type of its constituent pattern,
|
|
not a tuple type.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-is">'is' Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-is ::= 'is' <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p><tt>is</tt> patterns perform a type check equivalent to the
|
|
<tt>x is T</tt> <a href="#expr-cast">cast operator</a>. The pattern matches
|
|
if the runtime type of a value is of the given type. <tt>is</tt> patterns
|
|
are refutable and thus cannot appear in declarations.</p>
|
|
|
|
<pre class="example">
|
|
class B {}
|
|
class D1 : B {}
|
|
class D2 : B {}
|
|
|
|
var bs : B[] = [B(), D1(), D2()]
|
|
|
|
for b in bs {
|
|
switch b {
|
|
case is B:
|
|
println("B")
|
|
case is D1:
|
|
println("D1")
|
|
case is D2:
|
|
println("D2")
|
|
}
|
|
}
|
|
</pre>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-enum-element">Enum Element Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<pre class="grammar">
|
|
pattern-enum-element ::= <a href="#type-identifier">type-identifier</a>? '.' identifier <a href="#pattern-tuple">pattern-tuple</a>?
|
|
</pre>
|
|
|
|
<p>Enum element patterns match a value of
|
|
<a href="#type-enum">enum type</a> if the value matches the referenced
|
|
<tt>case</tt> of the enum. If the <tt>case</tt> has a type, the value
|
|
of that type can be matched against an optional subpattern.</pre>
|
|
|
|
<pre class="example">
|
|
enum HTMLTag {
|
|
case A(href:String)
|
|
case IMG(src:String, alt:String)
|
|
case BR
|
|
}
|
|
|
|
switch tag {
|
|
case .BR:
|
|
println("<br>")
|
|
case .IMG(var src, var alt):
|
|
println("<img src=\"\(escape(src))\" alt=\"\(escape(alt))\">")
|
|
case .A(var href):
|
|
println("<a href=\"\(escape(href))\">")
|
|
}
|
|
</pre>
|
|
|
|
<p>Enum element patterns are refutable and thus cannot appear in
|
|
declarations.
|
|
(They are currently considered refutable even if the enum contains only
|
|
a single <tt>case</tt>.)
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h3 id="pattern-expr">Expressions in Patterns</h3>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<p>Patterns may include arbitrary expressions as subpatterns. Expression
|
|
patterns are refutable and thus cannot appear in declarations. An expression
|
|
pattern is compared to its corresponding value using the <tt>~=</tt>
|
|
operator. The match succeeds if <tt>expr ~= value</tt> evaluates to true.
|
|
The standard library provides a default implementation of <tt>~=</tt>
|
|
using <tt>==</tt> equality; additionally, range objects may be matched
|
|
against integer and floating-point values. The <tt>~=</tt> operator may
|
|
be overloaded like any function.</p>
|
|
|
|
<pre class="example">
|
|
var point = (0, 0, 0)
|
|
switch point {
|
|
<i>// Equality comparison.</i>
|
|
case (0, 0, 0):
|
|
println("origin")
|
|
<i>// Range comparison.</i>
|
|
case (-10..10, -10..10, -10..10):
|
|
println("close to the origin")
|
|
default:
|
|
println("too far away")
|
|
}
|
|
|
|
<i>// Define pattern matching of an integer value to a string expression.</i>
|
|
func ~=(pattern:String, value:Int) -> Bool {
|
|
return pattern == "\(value)"
|
|
}
|
|
|
|
<i>// Now we can pattern-match strings to integers:</i>
|
|
switch point {
|
|
case ("0", "0", "0"):
|
|
println("origin")
|
|
default:
|
|
println("not the origin")
|
|
}
|
|
</pre>
|
|
|
|
<p>The order of evaluation of expressions in patterns, including whether an
|
|
expression is evaluated at all, is unspecified. The compiler is free to
|
|
reorder or elide expression evaluation in patterns to improve dispatch
|
|
efficiency. Expressions in patterns therefore cannot be relied on for
|
|
side effects.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="expr">Expressions</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<div class="commentary">
|
|
Support for user-defined operators causes some amount of parsing
|
|
to be delayed until after name resolution has occurred. Other
|
|
restrictions and disambiguations in the grammar permit the parser
|
|
to decide all other aspects of parsing, such as where statements
|
|
must be divided.<br><br>
|
|
|
|
Semicolons in C are generally just clutter. Swift generally tries
|
|
to define away the need for them.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr ::= expr-basic
|
|
expr ::= <a
|
|
href="#expr-trailing-closure">expr-trailing-closure</a> <a href="#expr-cast">expr-cast</a>?
|
|
|
|
expr-basic ::= expr-sequence <a href="#expr-cast">expr-cast</a>?
|
|
|
|
expr-sequence ::= <a href="#expr-unary">expr-unary</a> <a href="#expr-binary">expr-binary</a>*
|
|
|
|
expr-primary ::= <a href="#expr-literal">expr-literal</a>
|
|
expr-primary ::= <a href="#expr-identifier">expr-identifier</a>
|
|
expr-primary ::= <a href="#expr-super">expr-super</a>
|
|
expr-primary ::= <a href="#expr-closure">expr-closure</a>
|
|
expr-primary ::= <a href="#expr-anon-closure-arg">expr-anon-closure-arg</a>
|
|
expr-primary ::= <a href="#expr-paren">expr-paren</a>
|
|
expr-primary ::= <a href="#expr-delayed-identifier">expr-delayed-identifier</a>
|
|
|
|
expr-postfix ::= expr-primary
|
|
expr-postfix ::= expr-postfix <a href="#operator">operator-postfix</a>
|
|
expr-postfix ::= <a href="#expr-new">expr-new</a>
|
|
expr-postfix ::= <a href="#expr-dot">expr-dot</a>
|
|
expr-postfix ::= <a href="#expr-metatype">expr-metatype</a>
|
|
expr-postfix ::= <a href="#expr-subscript">expr-subscript</a>
|
|
expr-postfix ::= <a href="#expr-call">expr-call</a>
|
|
|
|
</pre>
|
|
|
|
<p>At the top level of the expression grammar, expressions are a
|
|
sequence of unary expressions joined by binary operators. When
|
|
parsing an expr, a binary operator immediately following an
|
|
expr-unary continues the expression, and the program is ill-formed
|
|
if it is not then followed by another expr-unary. This resolves
|
|
an ambiguity which could otherwise arise in statement contexts due
|
|
to semicolon elision.</p>
|
|
|
|
<pre class="example">
|
|
5 !- +~123 -+- ~+6
|
|
(foo)(())
|
|
bar(49+1)
|
|
baz()
|
|
</pre>
|
|
|
|
<p>A unary or binary expression may optionally be followed by a
|
|
<a href="#expr-cast">cast operator</a>.
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-binary">Binary Operators</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
Should this use the expr-identifier production to allow qualified
|
|
identifiers? This would allow "foo swift.+ bar". Is ADL or something
|
|
like it enough?<br><br>
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-binary ::= op-binary-or-ternary <a href="#expr-unary">expr-unary</a> expr-cast?
|
|
|
|
op-binary-or-ternary ::= <a href="#operator">operator-binary</a>
|
|
op-binary-or-ternary ::= '='
|
|
op-binary-or-ternary ::= '?' <a href="#expr">expr-sequence</a> ':'
|
|
|
|
expr-cast ::= 'is' <a href="#type">type</a>
|
|
expr-cast ::= 'as' '!' <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p>Infix binary expressions are not formed during parsing. Instead,
|
|
they are formed after name resolution by building a tree from an
|
|
operator-delimited sequence of unary expressions. Precedence and
|
|
associativity are determined by the <a href="#attribute-infix">infix</a>
|
|
attribute on the resolved names, which must fully agree.</p>
|
|
|
|
<p>If an operator is used as a binary operator, but name resolution
|
|
does not find at least one function of binary operator type, the
|
|
expression is ill-formed.</p>
|
|
|
|
<p>A simple example is:</p>
|
|
|
|
<pre class="example">
|
|
4 + 5 * 123
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-binary-builtin">Builtin Binary Operators</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<p>In addition to user-defined operators, a handful of builtin operators are
|
|
defined that parse inside binary expressions with predefined precedence and
|
|
associativity.
|
|
|
|
<h4 id="expr-assign">Assignment operator</h4>
|
|
|
|
<p>The assignment operator <tt>a = b</tt> updates the value of <tt>a</tt> with
|
|
the value of <tt>b</tt>. Its precedence is hardcoded as if declared as
|
|
follows:</p>
|
|
|
|
<pre class="example">
|
|
// Not valid Swift code
|
|
operator infix = {
|
|
precedence 90
|
|
associativity right
|
|
}
|
|
</pre>
|
|
|
|
The left-hand operand must be an lvalue, or a tuple of lvalues. Assigning to
|
|
a tuple of lvalues performs destructuring reassignment.
|
|
|
|
<pre class="example">
|
|
var (a, b) = (1, 2)
|
|
|
|
// Swap two values.
|
|
(a, b) = (b, a)
|
|
|
|
// Reassign two values.
|
|
(a, b) = (11, 22)
|
|
|
|
// Reassign two values by destructuring a tuple.
|
|
var tuple = (111, 222)
|
|
(a, b) = tuple
|
|
</pre>
|
|
|
|
<p>An assignment expression evaluates to void. Unlike C, productions such as
|
|
these are invalid:</p>
|
|
|
|
<pre class="example">
|
|
// Error: x = y doesn't return Bool
|
|
if x = y { }
|
|
|
|
// Error: (y = z) doesn't return Int
|
|
var x, y, z : Int
|
|
x = y = z
|
|
</pre>
|
|
|
|
<h4 id="expr-ternary">Ternary operator</h4>
|
|
|
|
<p>The ternary operator <tt>a ? b : c</tt> conditionally evaluates its middle
|
|
or right operand based on the value of its left operand. Its precedence is
|
|
hardcoded as if the middle <tt>? b :</tt> subexpression were a binary operator
|
|
declared as follows:</p>
|
|
|
|
<pre class="example">
|
|
// Not valid Swift code
|
|
operator infix ?...: {
|
|
precedence 100
|
|
associativity right
|
|
}
|
|
</pre>
|
|
|
|
<p>The subexpression to the left of the
|
|
'?' is evaluated, and is converted to 'Bool' using the result's
|
|
'getLogicValue' method if it is not already 'Bool'. If the condition is
|
|
true, the subexpression to the right of '?' is evaluated, and its result
|
|
becomes the result of the expression. If the
|
|
condition is false, the subexpression to the right of ':' is evaluated, and
|
|
its result becomes the result of the expression. Only one of the
|
|
'?' or ':' subexpressions will be evaluated. The results of the
|
|
'?' and ':' subexpressions must be implicitly convertible to a common type,
|
|
which becomes the type of the ternary expression.
|
|
|
|
<pre class="example">
|
|
x += b ? y : z
|
|
x += a ? b ? y : z : w
|
|
|
|
for i in 1..101 {
|
|
println(i % 15 ? "fizzbuzz"
|
|
: i % 3 == 0 ? "fizz"
|
|
: i % 5 == 0 ? "buzz"
|
|
: "\(i)")
|
|
}
|
|
</pre>
|
|
|
|
<h4 id="expr-cast">Cast operators</h4>
|
|
|
|
<p>Cast expressions influence the types of their subexpressions. They can appear
|
|
at the end of a binary operator sequence; their left operand is parsed as if
|
|
the cast operators were declared as follows:</p>
|
|
|
|
<pre class="example">
|
|
// Not valid Swift code
|
|
operator infix as {
|
|
precedence 95
|
|
associativity none
|
|
}
|
|
</pre>
|
|
|
|
<p>The right operand of all operators is parsed as a type.</p>
|
|
|
|
<ul>
|
|
<li><tt>x as! T</tt> will try to cast the value of the expression
|
|
<tt>x</tt> to a subtype of its compile-time type. The type of the value is
|
|
checked at runtime, and if the cast cannot succeed, the program terminates.
|
|
<tt>T</tt> must be a subtype of the compile-time type of <tt>x</tt>.
|
|
An example:
|
|
|
|
<pre class="example">
|
|
var b:B = new D
|
|
var d = b as! D
|
|
</pre>
|
|
|
|
<li><tt>x is T</tt> will query the type of the value of <tt>x</tt> at runtime.
|
|
<tt>T</tt> must be a subtype of the compile-time type of <tt>x</tt>.
|
|
If the runtime value of <tt>x</tt> is <tt>T</tt>, the <tt>is</tt> expression
|
|
evaluates to true; otherwise, it evaluates to false.
|
|
|
|
<pre class="example">
|
|
if b is D {
|
|
var d = b as! D
|
|
}
|
|
</pre>
|
|
</ul>
|
|
|
|
<p><tt>as!</tt> and <tt>is</tt> all parse a type for their
|
|
right-hand argument. They must be parenthesized if followed by subsequent
|
|
operators:
|
|
|
|
<pre class="example">
|
|
(b as! D).derivedMethod()
|
|
((B as! D) as! D2)
|
|
(b is D) ? (b as! D) : D()
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-unary">Unary Operators</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-unary ::= <a href="#operator">operator-prefix</a>* <a href="#expr">expr-postfix</a>
|
|
</pre>
|
|
|
|
<p>If an operator is used as a unary operator, but name resolution
|
|
does not find at least one function that takes a single argument, the
|
|
expression is ill-formed.</p>
|
|
|
|
<p>Simple examples:</p>
|
|
|
|
<pre class="example">
|
|
i = -j
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-literal">Literals</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
The type of a literal is inferred from its context, to allow things like "4"
|
|
to be compatible with any width integer type without 'promotion' rules or
|
|
casting. In ambiguous cases like "var x = 4", the literals are forced to
|
|
a default type specified by the standard library.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-literal ::= <a href="#integer_literal">integer_literal</a>
|
|
expr-literal ::= <a href="#floating_literal">floating_literal</a>
|
|
expr-literal ::= <a href="#character_literal">character_literal</a>
|
|
expr-literal ::= <a href="#string_literal">string_literal</a>
|
|
expr-literal ::= '__FILE__'
|
|
expr-literal ::= '__LINE__'
|
|
expr-literal ::= '__COLUMN__'
|
|
</pre>
|
|
|
|
<p>Numeric literals are either integer, floating point, character, or string
|
|
depending on its lexical form. The type of the literal is inferred
|
|
based on its context. If there is no contextual type information for an
|
|
expression, all unresolved types are inferred to 'IntegerLiteralType'
|
|
type, to 'FloatLiteralType', to 'CharacterLiteralType', and to
|
|
'StringLiteralType', respectively.
|
|
If a literal is used and these types are not defined, then the code is
|
|
malformed.</p>
|
|
|
|
<p>A literal is compatible with its inferred type if that type implements an
|
|
informal protocol required by literals. This informal protocol requires
|
|
that the type have an unambiguous "static" function defined whose
|
|
result type is the same as the inferred type, and that takes a single
|
|
argument that is either itself literal compatible, or is a <a
|
|
href="#builtin">builtin</a> integer type.</p>
|
|
|
|
<p>The '<tt>__FILE__</tt>', '<tt>__LINE__</tt>', and '<tt>__COLUMN__</tt>'
|
|
magic identifiers expand to a literal representation of their position in
|
|
the source code. '<tt>__FILE__</tt>' expands to a string literal;
|
|
'<tt>__LINE__</tt>' and '<tt>__COLUMN__</tt>' each expand to an integer
|
|
literal.</p>
|
|
|
|
<pre class="example">
|
|
<i>// File foo.swift</i>
|
|
|
|
var file = __FILE__ <i>// file : String = "foo.swift"</i>
|
|
var line = __LINE__ <i>// line : Int = 4</i>
|
|
var col = __COLUMN__ <i>// column : Int = 11</i>
|
|
</pre>
|
|
|
|
<p>If '<tt>__FILE__</tt>', '<tt>__LINE__</tt>', and/or '<tt>__COLUMN__</tt>'
|
|
are used as default argument values in a function declaration, they
|
|
instead expand to the source location of each function call that
|
|
instantiates the default argument.</p>
|
|
|
|
<pre class="example">
|
|
func log(message:String,
|
|
file:String = __FILE__,
|
|
line:Int = __LINE__) {
|
|
println("\(file):\(line): \(message)")
|
|
}
|
|
|
|
log("Orders received")
|
|
doIt()
|
|
log("Job's finished")
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-identifier">Identifiers</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-identifier ::= <a href="#identifier">identifier</a> <a href="#generic-args">generic-args</a>?
|
|
</pre>
|
|
|
|
<p>A raw identifier refers to a value found via <a
|
|
href="#namebind_value_lookup_unqual">unqualified value lookup</a>, and has
|
|
the type of the declaration returned by name lookup and overload
|
|
resolution. Value declarations are installed with <a
|
|
href="#decl-var">var</a> and the syntactic sugar forms like <a
|
|
href="decl-func">func</a> declarations.</p>
|
|
|
|
<p>If an identifier refers to a generic type, an instance of that generic may
|
|
be referenced by following the identifier with a list of type parameters
|
|
enclosed in angle brackets <tt><></tt>:</p>
|
|
|
|
<pre class="example">
|
|
<i>// A generic struct.</i>
|
|
struct Dict<K,V> {
|
|
init() {}
|
|
static func fromKeysAndValues(keys:K[], values:T[]) -> Dict<K,V> {}
|
|
}
|
|
|
|
<i>// Construct an instance of the generic struct.</i>
|
|
var foo = Dict<String, Int>()
|
|
<i>// Invoke a static method of an instance of the generic struct.</i>
|
|
var bar = Dict<String, Int>.fromKeysAndValues(
|
|
["zim", "zang", "zung"],
|
|
[ 123, 456, 789 ])
|
|
</pre>
|
|
|
|
<h4 id="expr-generic-disambiguation">Generic disambiguation</h4>
|
|
|
|
<p>Note that <tt><</tt> and <tt>></tt> are used as both angle brackets in
|
|
<a href="#expr-identifier">generic identifiers</a> and as characters in
|
|
<a href="#expr-binary">binary operator</a> names. Because of this, there are
|
|
potential parsing ambiguities. Swift uses a context-free heuristic to
|
|
determine whether to parse an expression involving <tt><</tt> and <tt>></tt>
|
|
as a generic parameter list or a binary operator:
|
|
|
|
<ul>
|
|
<li>When an <a href="#identifier">identifier</a> is followed by <tt><</tt>,
|
|
Swift attempts to parse starting from the <tt><</tt> as a
|
|
<a href="#type-identifier">generic parameter list</a>.
|
|
<li>If it succeeds in parsing a generic parameter list, it looks at the
|
|
token after the closing <tt>></tt>. If it sees one of the following tokens:
|
|
<blockquote>
|
|
<tt>( [ { } ] ) . , ;</tt>
|
|
</blockquote>
|
|
then the expression is parsed as a generic parameter list.
|
|
<li>If Swift cannot parse a generic parameter list after the <tt><</tt>,
|
|
or the matching <tt>></tt> is not followed by one of the above tokens,
|
|
the <tt><</tt> is parsed as an operator character.
|
|
</ul>
|
|
|
|
<p>These rules assume that, in most cases, generic type names will be used
|
|
in constructor expressions as in <tt>Foo<T>(x)</tt> or to access static
|
|
members as in <tt>Foo<T>.bar()</tt>. Referring to a generic metatype as a
|
|
value in an expression may require parentheses around the type name.
|
|
|
|
<pre class="example">
|
|
<i>// An operator that operates on metatypes.</i>
|
|
func [infix] +-+ <T, U>(t:T.metatype, u:U.metatype) -> Foo { }
|
|
|
|
var foo = (Dict<String, Int>) +-+ (Slice<Char>)
|
|
println(foo)
|
|
</pre>
|
|
|
|
<p>On the other hand, some expressions involving <tt><</tt> and
|
|
<tt>></tt> operators may misparse as generic arguments as well. These
|
|
can also be corrected by adding or removing parentheses.
|
|
|
|
<pre class="example">
|
|
func foo(x:Bool, y:Bool)
|
|
var a,b,c,d,e : Int
|
|
|
|
foo(a < b, c > (d + e)) // ERROR: Misparses as (a<b,c>)(d + e)
|
|
foo((a < b), c > (d + e)) // Force parsing as (a < b), (c > (d + e))
|
|
foo(a < b, c > d + e) // Also parses as (a < b), (c > (d + e))
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-super">Super</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-super ::= expr-super-method
|
|
expr-super ::= expr-super-subscript
|
|
expr-super ::= expr-super-constructor
|
|
|
|
expr-super-method ::= 'super' '.' <a href="#expr-identifier">expr-identifier</a>
|
|
expr-super-subscript ::= 'super' '[' <a href="#expr">expr</a> ']'
|
|
expr-super-constructor ::= 'super' '.' 'init'
|
|
</pre>
|
|
|
|
<p>The keyword <tt>super</tt> is used to refer to superclass members from
|
|
a subclass method. This can be used to access members of a superclass
|
|
overridden by the subclass. The following forms are allowed:
|
|
|
|
<ul>
|
|
<li>A superclass property or method can be accessed with the form
|
|
<tt>super.name</tt>.</li>
|
|
<li>A superclass subscript accessor can be accessed with the form
|
|
<tt>super[index]</tt>.</li>
|
|
<li>Within a constructor, a superclass constructor can be accessed with the
|
|
form <tt>super.init</tt>.</li>
|
|
</ul>
|
|
|
|
<p><tt>super</tt> expressions are invalid outside of a subclass method.
|
|
<tt>super.init</tt> is invalid outside of a subclass constructor.
|
|
<tt>super.init</tt> furthermore may only be called once per derived
|
|
constructor, and must be called before the derived constructor accesses
|
|
<tt>self</tt> or any instance variables.
|
|
</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-closure">Closure Expression</h3>
|
|
<!-- ===================================================================== -->
|
|
<pre class="grammar">
|
|
expr-closure ::= '{' closure-signature? <a href="#brace-item-list">brace-item-list</a> '}'
|
|
|
|
closure-signature ::= <a href="#pattern-tuple">pattern-tuple</a> <a href="#func-signature">func-signature-result</a>? 'in'
|
|
closure-signature ::= <a href="#identifier">identifier</a> (',' <a href="#identifier">identifier</a>*) <a href="#func-signature">func-signature-result</a>? 'in'
|
|
|
|
</pre>
|
|
|
|
<p>A closure defines an anonymous function as an expression. Like a
|
|
<a href="#decl-func">func</a> declaration, a closure has parameters,
|
|
a return type, and some number of statements that are executed when
|
|
the closure is called. Like local functions, closures can capture
|
|
values from its enclosing function and closure scopes. Closures are
|
|
often used in lieu of local functions when the function name would
|
|
only be used once, to be called by some other function. As a syntax
|
|
optimization, when the closure contains only a single expression, it's
|
|
value is used as the result of the closure. Thus, the closure <code>{
|
|
5 }</code> is equivalent to <code>{ return 5 }</code>.</p>
|
|
|
|
<p>Unlike <a href="#decl-func">func</a>
|
|
declarations, the return type, parameter types, and even the <a
|
|
href="#expr-anon-closure-arg">names of parameters</a> can be
|
|
omitted from the definition of the closure, making it a concise
|
|
syntax for small closures. In such cases, the context in which the
|
|
closure is used must provide information about the parameter and
|
|
return types. In the special case where the closure consists of only
|
|
a single expression, that expression participates in the
|
|
type checking of its context. </p>
|
|
|
|
<pre class="example">
|
|
<i>// Takes a closure that it calls to determine an ordering relation.</i>
|
|
func magic(val : Int, predicate : (a : Int, b : Int) -> Bool)
|
|
|
|
func f() {
|
|
<i>// Compare one way. Closure is inferred to return Bool and take two ints</i>
|
|
<i>// from the argument context. This same information infers that $0 and $1</i>
|
|
<i>// both have type 'Int'.</i>
|
|
magic(42, { $0 < $1 })
|
|
|
|
<i>// Compare the other way way.</i>
|
|
magic(42, { $1 < $0 })
|
|
|
|
<i>// Provide parameter names, but infer the types.</i>
|
|
magic(42, { x, y in y < x })
|
|
|
|
<i>// Provide parameter names and types.</i>
|
|
magic(42, { (x : Int, y : Int) in y < x })
|
|
|
|
<i>// Provide parameter names and types, and return type, with multiple statements.</i>
|
|
magic(42, { (x : Int, y : Int) -> Bool in
|
|
print("Comparing \(x) to \(y).\n")
|
|
return y < x
|
|
})
|
|
|
|
<i>// Error, not enough context to infer the type of $0.</i>
|
|
var x = { $0 }
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-anon-closure-arg">Anonymous Closure Arguments</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-anon-closure-arg ::= <a href="#dollarident">dollarident</a>
|
|
</pre>
|
|
|
|
<p>A use of an identifier whose name fits the "$[0-9]+" regular
|
|
expression is a reference to an anonymous closure argument that is formed when
|
|
the containing expression is <a href="#typecheck_anon">coerced into a closure
|
|
context</a>. All other dollar identifiers are invalid.</p>
|
|
|
|
<p>This can only be used in the body of a closure (<a
|
|
href="#expr-closure">expr-closure</a>) that does not have explicitly-specified parameters.
|
|
</p>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-delayed-identifier">Delayed Identifier Resolution</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
The ".bar" syntax was picked because it is related to the syntax of a fully
|
|
qualified "foo.bar" reference.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-delayed-identifier ::= '.' <a href="#identifier">identifier</a>
|
|
</pre>
|
|
|
|
<p>A delayed identifier expression refers to a constructor of a <a
|
|
href="type-enum">enum</a> type, without knowing which type it is referring
|
|
to. The expression is resolved to a constructor of a concrete type through
|
|
context sensitive type inference.</p>
|
|
|
|
<pre class="example">
|
|
enum Direction { case Up, Down }
|
|
func search(val : Int, direction : Direction)
|
|
|
|
func f() {
|
|
search(42, .Up)
|
|
search(17, .Down)
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-paren">Parenthesized Expressions</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-paren ::= '(' ')'
|
|
expr-paren ::= '(' expr-paren-element (',' expr-paren-element)* ')'
|
|
expr-paren-element ::= (<a href="#identifier">identifier</a> ':')? <a href="#expr">expr</a>
|
|
</pre>
|
|
|
|
<p>Parentheses expressions contain an (optionally empty) list of optionally
|
|
named values. Parentheses in an expression context denote one of two
|
|
things: 1) grouping parentheses, or 2) a tuple literal.</p>
|
|
|
|
<p>Grouping parentheses occur when there is exactly one value in the list and
|
|
that value does not have a name. In this case, the type of the parenthesis
|
|
expression is the type of the single value.</p>
|
|
|
|
<p>All other cases are tuple literals. The type of the expression is a tuple
|
|
type whose elements and order match that of the initializer. If there are
|
|
any named elements, those elements become names for the tuple type. A
|
|
parenthesis expression with no value has a type of the empty tuple.
|
|
</p>
|
|
|
|
<p>Some examples:</p>
|
|
|
|
<pre class="example">
|
|
<i>// Simple grouping parenthesis.</i>
|
|
var a = (4) <i>// Type = Int</i>
|
|
var b = (4+a) <i>// Type = Int</i>
|
|
|
|
<i>// Tuple literals.</i>
|
|
var c = () <i>// Type = ()</i>
|
|
var d = (4, 5) <i>// Type = (Int, Int)</i>
|
|
var e = (c, d) <i>// Type = ((), (Int, Int))</i>
|
|
|
|
var f = (x : 4, y : 5) <i>// Type = (x : Int, y : Int)</i>
|
|
var g = (4, y : 5, 6) <i>// Type = (Int, y : Int, Int)</i>
|
|
|
|
<i>// Named arguments to functions.</i>
|
|
func foo(a : Int, b : Int)
|
|
foo(b = 4, a = 1)
|
|
</pre>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-dot">Dot Expressions</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-dot ::= <a href="#expr">expr-postfix</a> '.' <a href="#dollarident">dollarident</a>
|
|
</pre>
|
|
|
|
<p>If the base expression has <a href="#type-tuple">tuple type</a>, then the
|
|
magic identifier "[0-9]+" accesses the specified anonymous member of the
|
|
tuple. Otherwise, this form is invalid.</p>
|
|
|
|
<pre class="grammar">
|
|
expr-dot ::= <a href="#expr">expr-postfix</a> '.' <a href="#expr-identifier">expr-identifier</a>
|
|
</pre>
|
|
|
|
<p>If the base expression has <a href="#type-tuple">tuple type</a> and if the
|
|
identifier is the name of a field in the tuple, then this is a reference to
|
|
the specified field.</p>
|
|
|
|
<p>Otherwise, <a href="#namebind_value_lookup_dot">dot name lookup</a> is
|
|
performed, and this expression is treated as function application. This
|
|
allows looking up members in modules, metatypes, etc.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-subscript">Subscript Expressions</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
There is no "built-in" semantics for subscripting. Rather, all
|
|
subscripting semantics is implemented via subscript declarations
|
|
in the library.
|
|
|
|
<br/>We require that the '[' not be the first token on a line, so that
|
|
a statement can begin with an array expression.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-subscript ::= <a href="#expr">expr-postfix</a> '[' <a href="#expr">expr</a> ']'
|
|
</pre>
|
|
|
|
<p>A subscript expression invokes a <a
|
|
href="#decl-subscript">subscript getter or setter</a> on the type
|
|
of the <tt>expr-postfix</tt>. The <tt>expr</tt> is used as the
|
|
subscript argument, which will be provided to either the getter or
|
|
setter depending on whether the subscript expression is used as an
|
|
rvalue (reading) or lvalue (writing), respectively. A subscript
|
|
expression that resolves to a subscript declaration with no setter
|
|
cannot be modified.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-new">New Expressions</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
It's not really clear what the behavior of multiple bounds should be.
|
|
|
|
<br/><br/>We should probably allow an initializer. The semantics would be
|
|
to evaluate that constructor for each element constructed.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-new ::= 'new' <a href="#type">type-identifier</a> expr-new-bounds
|
|
|
|
expr-new-bounds ::= expr-new-bound
|
|
expr-new-bounds ::= expr-new-bounds expr-new-bound
|
|
expr-new-bound ::= '[' <a href="#expr">expr?</a> ']'
|
|
</pre>
|
|
|
|
<p>Allocates and initializes a new array of objects. The first clause must
|
|
be an expression; subsequent bounds, if present, must be constant under
|
|
the <a href="#type-array">usual rules for array types</a>. The opening
|
|
square bracket must be on the same line as the type name.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-call">Function Application</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
expr-call ::= <a href="#expr">expr-postfix</a> <a href="#expr-paren">expr-paren</a>
|
|
</pre>
|
|
|
|
<p>The leading <tt>'('</tt> of the <tt>expr-paren</tt> must not be
|
|
the first token on a line. This greatly reduces the likelihood of
|
|
confusion from semicolon elision, without requiring feedback from
|
|
the typechecker or more aggressive whitespace sensitivity.</p>
|
|
|
|
<p>If the <tt>expr-postfix</tt> refers to a (possibly
|
|
parenthesized) name of a type, the <tt>expr-paren</tt> is first
|
|
coerced to the type named by <tt>expr-postfix</tt>. If that coercion
|
|
fails, then the <tt>expr-postfix</tt> refers to the set of
|
|
constructors for that type.</p>
|
|
|
|
<p>Simple examples:</p>
|
|
|
|
<pre class="example">
|
|
<i>// Application of an empty tuple to the function f.</i>
|
|
f()
|
|
<i>// Application of 4 to the function f.</i>
|
|
g(4)
|
|
|
|
<i>// Application of 4 to the function returned by h().</i>
|
|
var h : (Int) -> (Int) -> Int
|
|
...
|
|
h()(4)
|
|
|
|
<i>// Two separate statements</i>
|
|
i()
|
|
(j <+ 2)()
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="expr-trailing-closure">Trailing Closures</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
It is possible to model trailing closures as simply another way to
|
|
perform a function call, forgoing the syntactic transformation for
|
|
<a href="#expr-call">expr-call</a>, if functions meant to be used
|
|
with trailing closures are written as curried functions, e.g.,
|
|
<pre>
|
|
func map<T, U>(array : T[])(fn : (T) -> U) -> U[] { ... }
|
|
</pre>
|
|
There are two problems with this (admittedly simpler) design.
|
|
First, functions imported from C, C++, and Objective-C won't ever
|
|
be written in this curried syntax, so we would have to implement
|
|
redundant entry points to enable this syntax. Second, this design
|
|
forces the idea of currying front and center for Swift programmers
|
|
who otherwise wouldn't care, for mostly theoretical reasons.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
expr-trailing-closure ::= <a href="#expr">expr-postfix</a> <a href="#expr-closure">expr-closure</a>+
|
|
</pre>
|
|
|
|
<p>A postfix expression followed by a closure will be invoked with
|
|
the closure as its argument. This syntax is referred to as a
|
|
"trailing" closure, because the closure itself is outside the
|
|
parentheses used to call the expression. Trailing closures are
|
|
syntactic sugar that eliminates the awkwardness of closing a
|
|
function call with "})", where the "}" ends the closure and the ")"
|
|
ends the call.</p>
|
|
|
|
<p>Trailing closures use a simple syntactic translation, making them
|
|
purely syntactic sugar. If the postfix expression preceding the
|
|
trailing closure is an <a href="#expr-call">expr-call</a>, the
|
|
closure is added to the end of the <a
|
|
href="#expr-paren">expr-paren</a> of that call. Otherwise, the
|
|
postfix expression is (implicitly) called with the trailing closure
|
|
as its only argument.</p>
|
|
|
|
<pre class="example">
|
|
dispatch_async(q) {
|
|
print("Whenever you get around to it\n")
|
|
}
|
|
</pre>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id = "stmt">Statements</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<div class="commentary">
|
|
Statements can only exist in contexts that are themselves a stmt.
|
|
Statements have no type, they just induce control flow changes. We choose
|
|
to use constructs that will be familiar to a broad range of C/Java
|
|
programmers.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
stmt ::= <a href="#stmt-semicolon">stmt-semicolon</a>
|
|
stmt ::= <a href="#stmt-if">stmt-if</a>
|
|
stmt ::= <a href="#stmt-while">stmt-while</a>
|
|
stmt ::= <a href="#stmt-for-c-style">stmt-for-c-style</a>
|
|
stmt ::= <a href="#stmt-for-each">stmt-for-each</a>
|
|
stmt ::= <a href="#stmt-switch">stmt-switch</a>
|
|
stmt ::= stmt-control-transfer
|
|
|
|
stmt-control-transfer ::= <a href="#stmt-return">stmt-return</a>
|
|
stmt-control-transfer ::= <a href="#stmt-break">stmt-break</a>
|
|
stmt-control-transfer ::= <a href="#stmt-continue">stmt-continue</a>
|
|
stmt-control-transfer ::= <a href="#stmt-fallthrough">stmt-fallthrough</a>
|
|
</pre>
|
|
|
|
<p>Statements provide the control flow constructs of function bodies and
|
|
top-level code.</p>
|
|
|
|
<pre class="example">
|
|
<i>// A function with some statements.</i>
|
|
func fib(v : Int) -> Int {
|
|
if v < 2 {
|
|
return v
|
|
}
|
|
return fib(v-1)+fib(v-2)
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-semicolon">Semicolon Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
Allowing semicolons as statements causes us to allow semicolons as statement
|
|
separators as well. This, in turn, means that we don't reject code that has
|
|
semicolons after each statement, which will be common when people first
|
|
start getting used to Swift.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
stmt-semicolon ::= ';'
|
|
</pre>
|
|
|
|
<p>The semicolon statement has no effect.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-return">'return' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-return ::= 'return' <a href="#expr">expr</a>
|
|
stmt-return ::= 'return'
|
|
</pre>
|
|
|
|
<p>The return statement sets the return value of the current <a
|
|
href="#decl-func">func declaration</a> or <a href="#expr-closure">closure
|
|
expression</a> and transfers control out of the function. It sets the
|
|
return value by converting the specified expression result (or '()' if
|
|
none is specified) to the return type of the 'func'.
|
|
</p>
|
|
|
|
<p>The stmt-return grammar is ambiguous: "{ return 4 }" could be parsed as
|
|
{"return" "4"} or as a single statement. Ambiguity here is resolved toward
|
|
the first production, because control flow can't transfer to an
|
|
subexpression.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-break">'break' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-return ::= 'break'
|
|
</pre>
|
|
|
|
<p>The 'break' statement transfers control out of the enclosing 'for' loop or
|
|
'while' loop.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-continue">'continue' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-return ::= 'continue'
|
|
</pre>
|
|
|
|
<p>The 'continue' statement transfers control back to the start of the
|
|
enclosing 'for' loop or 'while' loop.</p>
|
|
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-if">'if' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
We require braces around the body of an 'if' for two reasons: first, it
|
|
eliminates the need for parentheses around the condition by making them
|
|
visually distinctive. Second, it will eliminate all the dithering about
|
|
whether and when people should, or should not, use braces for if bodies.
|
|
</div>
|
|
|
|
<pre class="grammar">
|
|
stmt-if ::= 'if' <a href="#expr">expr-basic</a> <a href="#brace-item-list">brace-item-list</a> stmt-if-else?
|
|
stmt-if-else ::= 'else' <a href="#brace-item-list">brace-item-list</a>
|
|
stmt-if-else ::= 'else' stmt-if
|
|
</pre>
|
|
|
|
<p>'if' statements provide a simple control transfer operations that evaluates
|
|
the condition, invokes the 'getLogicValue' member of the result if the result
|
|
not a 'Bool', then determines the direction of the branch based on the result.
|
|
(Internally, the standard library type 'Bool' has a getLogicValue member that
|
|
returns a 'Builtin.Int1'.) It is an error if the type of the expression is
|
|
context-dependent or some non-Bool type.
|
|
</p>
|
|
|
|
<p>Some examples include:</p>
|
|
|
|
<pre class="example">
|
|
if true {
|
|
/*...*/
|
|
}
|
|
|
|
if X == 4 {
|
|
} else {
|
|
}
|
|
|
|
if X == 4 {
|
|
} else if X == 5 {
|
|
} else {
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-while">'while' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-while ::= 'while' <a href="#expr">expr-basic</a> <a href="#brace-item-list">brace-item-list</a>
|
|
</pre>
|
|
|
|
<p>'while' statements provide simple loop construct which (on each iteration
|
|
of the loop) evalutes the condition, invokes the 'getLogicValue' member of
|
|
the result if the result not a 'Bool', then determines whether to keep
|
|
looping. (Internally, the standard library type 'Bool' has a getLogicValue
|
|
member that returns a 'Builtin.Int1'.) It is an error if the type of
|
|
the expression is context-dependent or some non-Bool type.
|
|
</p>
|
|
|
|
<p>Some examples include:</p>
|
|
|
|
<pre class="example">
|
|
while true {
|
|
/*...*/
|
|
}
|
|
|
|
while X == 4 {
|
|
X = 3
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-do-while">'do-while' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-do-while ::= 'do' <a href="#brace-item-list">brace-item-list</a> 'while' '<a href="#expr">expr</a>
|
|
</pre>
|
|
|
|
<p>'do-while' statements provide simple loop construct which (on each
|
|
iteration of the loop) evaluates the body, then evaluates the condition,
|
|
invoking the 'getLogicValue' member of the result if the result not a 'Bool',
|
|
then determines whether to keep looping. (Internally, the standard library
|
|
type 'Bool' has a getLogicValue member that returns a 'Builtin.Int1'). It is
|
|
an error if the type of the expression is context-dependent or some non-Bool
|
|
type.
|
|
</p>
|
|
|
|
<p>Some examples include:</p>
|
|
|
|
<pre class="example">
|
|
do {
|
|
/*...*/
|
|
} while true
|
|
|
|
do {
|
|
X = 3
|
|
} while X == 4
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-for-c-style">C-Style 'for' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-for-c-style ::= 'for' stmt-for-c-style-init? ';' <a href="#expr">expr</a>? ';' expr-basic? <a href="#brace-item-list">brace-item-list</a>
|
|
stmt-for-c-style ::= 'for' '(' stmt-for-c-style-init? ';' <a href="#expr">expr</a>? ';' expr-basic? ')' <a href="#brace-item-list">brace-item-list</a>
|
|
stmt-for-c-style-init ::= <a href="#decl-var">decl-var</a>
|
|
stmt-for-c-style-init ::= expr
|
|
</pre>
|
|
|
|
<p>C-Style 'for' statements provide simple loop construct which evaluates the
|
|
first part (the initializer) before entering the loop, then evalutes the
|
|
second condition as a logic value to determines whether to keep looping.
|
|
The third condition is executed at the end of the loop. All three are
|
|
evaluated in a new scope that surrounds the for statement.
|
|
</p>
|
|
|
|
<p>Some examples include:</p>
|
|
|
|
<pre class="example">
|
|
for i = 0; i != 10; ++i {
|
|
/*...*/
|
|
}
|
|
|
|
for (i = 0; i != 10; ++i) {
|
|
/*...*/
|
|
}
|
|
|
|
for var (i,j) = (0,1); i != 10; ++i {
|
|
/*...*/
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-for-each">'for-each' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-for-each ::= 'for' <a href="#pattern">pattern</a> 'in' <a href="#expr">expr-basic</a> <a href="#brace-item-list">brace-item-list</a>
|
|
</pre>
|
|
|
|
<p>Enumerator-based 'for' statements provide enumeration over the values in a
|
|
container. The <tt>expr</tt> is either a container or an enumerator; and
|
|
respectively, it either conforms to the formal Enumeration or formal Enumerator
|
|
protocol.
|
|
|
|
<p>Note that each iteration of the loop declares a distinct variable for each
|
|
variable in the pattern. For example, in a loop like "for i in 0..10",
|
|
if i is captured inside the loop, each iteration captures a different "i",
|
|
so there would be a total of ten versions generated each time the loop
|
|
runs.</p>
|
|
|
|
<p>Some examples include:</p>
|
|
|
|
<pre class="example">
|
|
for i in 0..100 {
|
|
println(String(i));
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-switch">'switch' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-switch ::= 'switch' <a href="#expr">expr-basic</a> '{' stmt-switch-case* '}'
|
|
stmt-switch-case ::= (case-label+ | default-label) <a href="#brace-item-list">brace-item</a>*
|
|
|
|
case-label ::= 'case' <a href="#pattern">pattern</a> (',' pattern)* ('where' expr)? ':'
|
|
default-label ::= 'default' ':'
|
|
</pre>
|
|
|
|
<p>'switch' statements branch on the value of an expression by
|
|
<a href="#pattern">pattern matching</a>.
|
|
The subject expression of the switch is evaluated and tested against the
|
|
patterns in its <tt>case</tt> labels in source order. When a pattern is
|
|
found that matches the value, control is transferred into the matching
|
|
<tt>case</tt> block. <tt>case</tt> labels may declare multiple patterns
|
|
separated by commas, and multiple <tt>case</tt> labels may cover a
|
|
case block. Case labels may optionally specify a <i>guard</i> expression,
|
|
introduced by the <tt>where</tt> keyword; if present, control is
|
|
transferred to the case only if the subject value both matches one of its
|
|
patterns and the guard expression evaluates to true. Patterns are tested
|
|
"as if" in source order; if multiple cases can match a value, control is
|
|
transferred only to the first matching case. The actual execution order
|
|
of pattern matching operations, and in particular the evaluation order of
|
|
<a href="#pattern-expr">expression patterns</a>, is unspecified.</p>
|
|
|
|
<p>A switch may also contain a <tt>default</tt> block. If present, it
|
|
receives control if no cases match the subject value. The <tt>default</tt>
|
|
block must appear at the end of the switch and must be the only label for
|
|
its block. (<tt>default</tt> is equivalent to a final <tt>case _</tt>
|
|
pattern.) Switches are required to be exhaustive; either the contained case
|
|
patterns must cover every possible value of the subject's type, or else
|
|
an explicit <tt>default</tt> block must be specified to handle uncovered
|
|
cases.</p>
|
|
|
|
<p>Every case and default block has its own
|
|
scope. Declarations within a case or default block are only visible within
|
|
that block. Case patterns may bind variables using the
|
|
<a href="#pattern-var"><tt>var</tt> keyword</a>; those variables are also
|
|
scoped into the corresponding case block, and may be referenced in the
|
|
<tt>where</tt> guard for the case label. However, if a case block matches
|
|
multiple patterns, none of those patterns may contain variable bindings.</p>
|
|
|
|
<p>Control does not implicitly 'fall through' from one case block to
|
|
the next. <a href="#stmt-fallthrough"><tt>fallthrough</tt> statements</a>
|
|
may explicitly transfer control among case blocks.
|
|
<a href="#stmt-break"><tt>break</tt></a> and
|
|
<a href="#stmt-continue"><tt>continue</tt></a>
|
|
within a switch will break or continue out of an enclosing 'while' or
|
|
'for' loop, not out of the 'switch' itself.</p>
|
|
|
|
<pre class="example">
|
|
func classifyPoint(point:(Int, Int)) {
|
|
switch point {
|
|
case (0, 0):
|
|
println("origin")
|
|
|
|
case (_, 0):
|
|
println("on the x axis")
|
|
|
|
case (0, _):
|
|
println("on the y axis")
|
|
|
|
case (var x, var y) where x == y:
|
|
println("on the y = x diagonal")
|
|
|
|
case (var x, var y) where -x == y:
|
|
println("on the y = -x diagonal")
|
|
|
|
case (var x, var y):
|
|
println("length \(sqrt(x*x + y*y))")
|
|
}
|
|
}
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stmt-fallthrough">'fallthrough' Statement</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
stmt-fallthrough ::= 'fallthrough'
|
|
</pre>
|
|
|
|
<p><tt>fallthrough</tt> transfers control from a <tt>case</tt> block of a
|
|
<a href="#stmt-switch"><tt>switch</tt> statement</a> to the next
|
|
<tt>case</tt> or <tt>default</tt> block within the switch. It may only
|
|
appear inside a <tt>switch</tt>. <tt>fallthrough</tt> cannot be used in
|
|
the final block of a <tt>switch</tt>. It also cannot transfer control into
|
|
a <tt>case</tt> block whose pattern contains
|
|
<a href="#pattern-var"><tt>var</tt> bindings</a>.</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2>Protocols</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2>Objects</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2>Generics</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="generic-params">Generic Parameters</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
generic-params ::= '<' generic-param (',' generic-param)* where-clause? '>'
|
|
|
|
generic-param ::= identifier
|
|
generic-param ::= identifier ':' <a href="#type-identifier">type-identifier</a>
|
|
generic-param ::= identifier ':' <a href="#type-composition">type-composition</a>
|
|
|
|
<a id="where-clause">where-clause</a> ::= 'where' requirement (',' requirement) *
|
|
|
|
requirement ::= conformance-requirement
|
|
::= same-type-requirement
|
|
|
|
conformance-requirement ::= <a href="#type-identifier">type-identifier</a> ':' <a href="#type-identifier">type-identifier</a>
|
|
conformance-requirement ::= <a href="#type-identifier">type-identifier</a> ':' <a href="#type-composition">type-composition</a>
|
|
|
|
same-type-requirement ::= <a href="#type-identifier">type-identifier</a> '==' <a href="#type-identifier">type-identifier</a>
|
|
</pre>
|
|
|
|
<p>A generic function or type is parameterized by a given set of
|
|
generic parameters. The generic parameters each have a name as well
|
|
as some set of requirements that specify the capabilities that any
|
|
corresponding generic argument might have. For example, the generic
|
|
parameter <code>T : Printable</code> requires that any generic
|
|
argument substituted for the generic parameter <code>T</code>
|
|
conform to the protocol <code>Printable</code>. Similarly, a generic
|
|
parameter <code>U : SomeClass</code> requires that any generic
|
|
argument substituted for the generic parameter <code>U</code>
|
|
inherit from the class <code>SomeClass</code>.
|
|
|
|
<p>Additional requirements on generic parameters and associated types
|
|
of generic parameters can be introduced via the "where" clause,
|
|
which can include additional protocol-conformance requirements
|
|
(e.g., the generic parameter list <code><T where T :
|
|
Printable></code>, which is equivalent to <code><T :
|
|
Printable></code>), as well as same-type requirements that
|
|
require two types to be identical (e.g., <code><T : Collection, U
|
|
: Collection where T.Element == U.Element></code>).
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="generic-args">Generic Arguments</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<pre class="grammar">
|
|
generic-args ::= '<' generic-arg (',' generic-arg)* '>'
|
|
|
|
generic-arg ::= <a href="#type">type</a>
|
|
</pre>
|
|
|
|
<p>Generic argument lists specify the generic arguments to be provided
|
|
to a generic type or function, which replace the generic parameters
|
|
of that type or function to produce a specialized version of that
|
|
type or function. For example, given a generic class:
|
|
|
|
<pre class="example">
|
|
class Dictionary<Key : Hashable, Value> { /* ... */ }
|
|
</pre>
|
|
|
|
<p>The type <code>Dictionary<String, Int></code>, replaces the
|
|
generic parameter <code>Key</code> with <code>String</code> and the
|
|
generic parameter <code>Value</code> with <code>Int</code>. Each
|
|
generic argument must satisfy all of the requirements of its
|
|
corresponding generic parameter (e.g., <code>String</code> must
|
|
conform to the <code>Hashable</code> protocol), and all generic
|
|
arguments, when taken together, must satisfy the additional
|
|
requirements specified in the <code>where</code> clause.
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="namebind">Name Binding</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<p>Name binding in swift is performed in different ways depending on what
|
|
language entity is being considered:</p>
|
|
|
|
<p>Value names (for <a
|
|
href="#decl-var">var</a> and <a href="#decl-func">func</a> declarations) and
|
|
type names (for <a href="#decl-typealias">typealias</a>, <a
|
|
href="#decl-enum">enum</a>, and <a href="#decl-struct">struct</a>
|
|
declarations) follow the same <a href="#namebind_scope">scope</a> and
|
|
<a href="#namebind_typevalue_lookup">name lookup</a> rules as described below.
|
|
</p>
|
|
|
|
<p>tuple element names</p>
|
|
|
|
<p>scope within enum decls</p>
|
|
|
|
<p>Context sensitive member references are resolved <a
|
|
href="#typecheck_context">during type checking</a>.</p>
|
|
|
|
<h3 id="namebind_scope">Scopes for Type and Value Names</h3>
|
|
|
|
|
|
<h3 id="namebind_value_lookup_unqual">Name Lookup Unqualified Value Names</h3>
|
|
<h3 id="namebind_value_lookup_dot">"dot" Name Lookup Value Names</h3>
|
|
|
|
<h3 id="namebind_typevalue_lookup">Name Lookup for Type and Value Names</h3>
|
|
|
|
<p>Basic algo:</p>
|
|
|
|
<ul>
|
|
<li>Search the current scope tree for a local name. Local names cannot be
|
|
forward referenced.</li>
|
|
<li>Bind to names defined in the current component, including the current
|
|
translation unit. TODO: is this a good thing? We could require explicit
|
|
imports if we wanted to.</li>
|
|
<li>Bind to identifiers that are imported with an import directive. Imports
|
|
are searched in order of introduction (top-down). The location of an
|
|
import directive in a file (e.g. between func decls) does not affect name
|
|
lookup, but the order of imports w.r.t. each other does.</li>
|
|
</ul>
|
|
|
|
<p>Shadowing: Given a ValueDecl D1 in the current module and a ValueDecl D2
|
|
in an imported module with the same name and a member of the same type (if
|
|
relevant): 1. If D1 is a TypeDecl, D2 is shadowed. 2. If neither D1 nor D2
|
|
is a TypeDecl, and they have the same type, D2 is shadowed. If a
|
|
declaration in an imported module is shadowed by any declaration in the
|
|
current module, it is not found by unqualified global lookup or lookup for
|
|
members of a type.</p>
|
|
|
|
<h3 id="namebind_dot">Name Lookup for Dot Expressions</h3>
|
|
|
|
<p>
|
|
<a href="#expr-dot">Dot Expressions</a> bind to name of tuple elements.
|
|
</p>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="typecheck">Type Checking</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<p>
|
|
Binary expressions, function application, etc.
|
|
</p>
|
|
|
|
<h3 id="typecheck_conversions">Standard Conversions</h3>
|
|
|
|
<!--
|
|
Consider foo(4, 5) when foo is declared to take ((Int,Int=3), Int=6). This
|
|
could be parsed as either ((4,5), 6) or ((4,3),5), but the later one is
|
|
the "right" answer.
|
|
-->
|
|
|
|
<h3 id="typecheck_anon">Anonymous Argument Resolution</h3>
|
|
<h3 id="typecheck_context">Context Sensitive Type Resolution</h3>
|
|
|
|
<!-- ********************************************************************* -->
|
|
<h2 id="stdlib">Standard Library</h2>
|
|
<!-- ********************************************************************* -->
|
|
|
|
<div class="commentary">
|
|
It would be really great to have literate swift code someday, that way
|
|
this could be generated directly from the code. This would also be powerful
|
|
for Swift library developers to be able to depend on being available and
|
|
standardized.
|
|
</div>
|
|
|
|
<p>This describes some of the standard swift code as it is being built up.
|
|
Since Swift is designed to give power to the library developers, much of
|
|
what is normally considered the "language" is actually just implemented in
|
|
the library.</p>
|
|
|
|
<p>All of this code is published by the 'swift' module, which is
|
|
implicitly imported into each translation unit, unless some sort of pragma
|
|
in the code (attribute on an import?) is used to change or disable this
|
|
behavior.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="builtin">Builtin Module</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<p>In the initial Swift implementation, a module named <tt>Builtin</tt> is
|
|
imported into every file. Its declarations can only be found by <a
|
|
href="#expr-dot">dot syntax</a>. It provides access to a small
|
|
number of primitive representation types and operations defined over them
|
|
that map directly to LLVM IR.</p>
|
|
|
|
<p>The existance of and details of this module are a private implementation
|
|
detail used by our implementation of the standard library. Swift code
|
|
outside the standard library should not be aware of this library, and an
|
|
independent implementation of the swift standard library should be
|
|
allowed to be implemented without the builtin library if it desires.</p>
|
|
|
|
<p>For reference below, the description of the standard library uses the
|
|
"Builtin." namespace to refer to this module, but independent
|
|
implementations could use another implementation if they so desire.</p>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stdlib-simple-types">Simple Types</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<h4 id="stdlib-Void">Void</h4>
|
|
|
|
<pre class="stdlib">
|
|
<i>// Void is just a type alias for the empty tuple.</i>
|
|
typealias Void = ()
|
|
</pre>
|
|
|
|
<div class="commentary">
|
|
Having a single standardized integer type that can be used by default
|
|
everywhere is important. One advantage Swift has is that by the time it is
|
|
in widespread use, 64-bit architectures will be pervasive, and the LLVM
|
|
optimizer should grow to be good at shrinking 64-bit integers to 32-bit in
|
|
many cases for those 32-bit architectures that persist.
|
|
</div>
|
|
|
|
<h4 id="stdlib-Int">Int, Int8, Int16, Int32, Int64</h4>
|
|
|
|
<pre class="stdlib">
|
|
<i>// Fixed size types are simple structs of the right size.</i>
|
|
struct Int8 { value : Builtin.Int8 }
|
|
struct Int16 { value : Builtin.Int16 }
|
|
struct Int32 { value : Builtin.Int32 }
|
|
struct Int64 { value : Builtin.Int64 }
|
|
struct Int128 { value : Builtin.Int128 }
|
|
|
|
<i>// Int is just an alias for the 64-bit integer type.</i>
|
|
typealias Int = Int64
|
|
</pre>
|
|
|
|
<h4 id="stdlib-Int">Int, Int8, Int16, Int32, Int64</h4>
|
|
<pre class="stdlib">
|
|
struct Float { value : Builtin.FPIEEE32 }
|
|
struct Double { value : Builtin.FPIEEE64 }
|
|
</pre>
|
|
|
|
<h4 id="stdlib-Bool">Bool, true, false</h4>
|
|
|
|
<pre class="stdlib">
|
|
<i>// Bool is a simple enum.</i>
|
|
enum Bool {
|
|
true, false
|
|
}
|
|
|
|
<i>// Allow true and false to be used unqualified.</i>
|
|
var true = Bool.true
|
|
var false = Bool.false
|
|
</pre>
|
|
|
|
<!-- ===================================================================== -->
|
|
<h3 id="stdlib-arithmetic">Arithmetic and Logical Operations</h3>
|
|
<!-- ===================================================================== -->
|
|
|
|
<div class="commentary">
|
|
This is all eagerly awaiting the day when we have generics and overloading.
|
|
For now, Int is the only arithmetic type :)
|
|
</div>
|
|
|
|
<h4 id="stdlib-arithmetic">Arithmetic Operators</h4>
|
|
|
|
<pre class="stdlib">
|
|
<i>// Simple binary operators, following the same precedence as C.</i>
|
|
func [infix_left=200] * (lhs: Int, rhs: Int) -> Int
|
|
func [infix_left=200] / (lhs: Int, rhs: Int) -> Int
|
|
func [infix_left=200] % (lhs: Int, rhs: Int) -> Int
|
|
func [infix_left=190] + (lhs: Int, rhs: Int) -> Int
|
|
func [infix_left=190] - (lhs: Int, rhs: Int) -> Int
|
|
<i>// In C, <<, >> is 180.</i>
|
|
</pre>
|
|
|
|
<h4 id="stdlib-comparison">Relational and Equality Operators</h4>
|
|
|
|
<pre class="stdlib">
|
|
func [infix_left=170] < : (lhs : Int, rhs : Int) -> Bool
|
|
func [infix_left=170] > : (lhs : Int, rhs : Int) -> Bool
|
|
func [infix_left=170] <= : (lhs : Int, rhs : Int) -> Bool
|
|
func [infix_left=170] >= : (lhs : Int, rhs : Int) -> Bool
|
|
func [infix_left=160] == : (lhs : Int, rhs : Int) -> Bool
|
|
func [infix_left=160] != : (lhs : Int, rhs : Int) -> Bool
|
|
<i>// In C, bitwise logical operators are 130,140,150.</i>
|
|
</pre>
|
|
|
|
<h4 id="stdlib-short-circuit-logical">Short Circuiting Logical Operators</h4>
|
|
|
|
<pre class="stdlib">
|
|
func [infix_left=120] && (lhs: Bool, rhs: ()->Bool) -> Bool
|
|
func [infix_left=110] || (lhs: Bool, rhs: ()->Bool) -> Bool
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<i>// In C, 100 is ?:</i>
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<i>// In C, 90 is =, *=, += etc.</i>
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</pre>
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