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22 lines
733 B
Swift
22 lines
733 B
Swift
// RUN: not %target-swift-frontend %s -typecheck
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protocol Fixpoint {
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typealias Algebra : Algebraic
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init(_ : Algebra)
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var out: Algebra { get }}
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protocol Algebraic {typealias Recur}
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enum Expr<T>: Algebraic {
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typealias Recur = T
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case Null
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func map<U>(transform: T -> U) -> Expr<U> {return .Null}
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}
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func out<Fix: Fixpoint>(v: Fix) -> Fix.Recur {return v.out}
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func cata<T, Fix: Fixpoint where Fix.Recur == Expr<Fix>>(f: Expr<T> -> T)(_ term: Fix) -> T {
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return f({ $0.map(cata(f)) }(out(term)))
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}
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struct Term: Fixpoint, CustomDebugStringConvertible {
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typealias Algebra = Expr<Term>
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var debugDescription: String {return cata(Term.toDebugString)(self)}
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static func toDebugString(expression: Expr<String>) -> String {return ""}
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}
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