Now accepted as
[SE-0489](https://github.com/ZevEisenberg/swift-evolution/blob/main/proposals/0489-codable-error-printing.md).
# To Do
- [x] confirm which version of Swift to use for the availability
annotations. Probably 6.3 at time of writing.
# Context
Re: [Swift forum
post](https://forums.swift.org/t/the-future-of-serialization-deserialization-apis/78585/77),
where a discussion about future serialization tools in Swift prompted
Kevin Perry to suggest that some proposed changes could actually be made
in today's stdlib.
# Summary of Changes
Conforms `EncodingError` and `DecodingError` to
`CustomDebugStringConvertible` and adds a more-readable
`debugDescription`.
# Future Directions
This is a pared-down version of some experiments I did in
[UsefulDecode](https://github.com/ZevEisenberg/UsefulDecode). The
changes in this PR are the best I could do without changing the public
interface of `DecodingError` and `EncodingError`, and without modifying
the way the `JSON`/`PropertyList` `Encoder`/`Decoder` in Foundation
generate their errors' debug descriptions.
In the above-linked
[UsefulDecode](https://github.com/ZevEisenberg/UsefulDecode) repo, when
JSON decoding fails, I go back and re-decode the JSON using
`JSONSerialization` in order to provide more context about what failed,
and why. I didn't attempt to make such a change here, but I'd like to
discuss what may be possible.
# Examples
To illustrate the effect of the changes in this PR, I removed my changes
to stdlib/public/core/Codable.swift and ran my new test cases again.
Here are the resulting diffs.
##
`test_encodingError_invalidValue_nonEmptyCodingPath_nilUnderlyingError`
### Before
`invalidValue(234, Swift.EncodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription: "You
cannot do that!", underlyingError: nil))`
### After
`EncodingError.invalidValue: 234 (Int). Path: first.second[2]. Debug
description: You cannot do that!`
## `test_decodingError_valueNotFound_nilUnderlyingError`
### Before
`valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "firstName", intValue: nil)],
debugDescription: "Description for debugging purposes", underlyingError:
nil))`
### After
`DecodingError.valueNotFound: Expected value of type String but found
null instead. Path: [0].firstName. Debug description: Description for
debugging purposes`
## `test_decodingError_keyNotFound_nonNilUnderlyingError`
### Before
`keyNotFound(GenericCodingKey(stringValue: "name", intValue: nil),
Swift.DecodingError.Context(codingPath: [GenericCodingKey(stringValue:
"0", intValue: 0), GenericCodingKey(stringValue: "address", intValue:
nil), GenericCodingKey(stringValue: "city", intValue: nil)],
debugDescription: "Just some info to help you out", underlyingError:
Optional(main.GenericError(name: "hey, who turned out the lights?"))))`
### After
`DecodingError.keyNotFound: Key \'name\' not found in keyed decoding
container. Path: [0].address.city. Debug description: Just some info to
help you out. Underlying error: GenericError(name: "hey, who turned out
the lights?")`
## `test_decodingError_typeMismatch_nilUnderlyingError`
### Before
`typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "address", intValue: nil),
GenericCodingKey(stringValue: "city", intValue: nil),
GenericCodingKey(stringValue: "birds", intValue: nil),
GenericCodingKey(stringValue: "1", intValue: 1),
GenericCodingKey(stringValue: "name", intValue: nil)], debugDescription:
"This is where the debug description goes", underlyingError: nil))`
### After
`DecodingError.typeMismatch: expected value of type String. Path:
[0].address.city.birds[1].name. Debug description: This is where the
debug description goes`
## `test_decodingError_dataCorrupted_nonEmptyCodingPath`
### Before
`dataCorrupted(Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription:
"There was apparently some data corruption!", underlyingError:
Optional(main.GenericError(name: "This data corruption is getting out of
hand"))))`
### After
`DecodingError.dataCorrupted: Data was corrupted. Path: first.second[2].
Debug description: There was apparently some data corruption!.
Underlying error: GenericError(name: "This data corruption is getting
out of hand")`
## `test_decodingError_valueNotFound_nonNilUnderlyingError`
### Before
`valueNotFound(Swift.Int, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "population", intValue: nil)],
debugDescription: "Here is the debug description for value-not-found",
underlyingError: Optional(main.GenericError(name: "these aren\\\'t the
droids you\\\'re looking for"))))`
### After
`DecodingError.valueNotFound: Expected value of type Int but found null
instead. Path: [0].population. Debug description: Here is the debug
description for value-not-found. Underlying error: GenericError(name:
"these aren\\\'t the droids you\\\'re looking for")`
##
`test_encodingError_invalidValue_emptyCodingPath_nonNilUnderlyingError`
### Before
`invalidValue(345, Swift.EncodingError.Context(codingPath: [],
debugDescription: "You cannot do that!", underlyingError:
Optional(main.GenericError(name: "You really cannot do that"))))`
### After
`EncodingError.invalidValue: 345 (Int). Debug description: You cannot do
that!. Underlying error: GenericError(name: "You really cannot do
that")`
## `test_decodingError_typeMismatch_nonNilUnderlyingError`
### Before
`typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "address", intValue: nil),
GenericCodingKey(stringValue: "1", intValue: 1),
GenericCodingKey(stringValue: "street", intValue: nil)],
debugDescription: "Some debug description", underlyingError:
Optional(main.GenericError(name: "some generic error goes here"))))`
### After
`DecodingError.typeMismatch: expected value of type String. Path:
[0].address[1].street. Debug description: Some debug description.
Underlying error: GenericError(name: "some generic error goes here")`
## `test_encodingError_invalidValue_emptyCodingPath_nilUnderlyingError`
### Before
`invalidValue(123, Swift.EncodingError.Context(codingPath: [],
debugDescription: "You cannot do that!", underlyingError: nil))`
### After
`EncodingError.invalidValue: 123 (Int). Debug description: You cannot do
that!`
## `test_decodingError_keyNotFound_nilUnderlyingError`
### Before
`keyNotFound(GenericCodingKey(stringValue: "name", intValue: nil),
Swift.DecodingError.Context(codingPath: [GenericCodingKey(stringValue:
"0", intValue: 0), GenericCodingKey(stringValue: "address", intValue:
nil), GenericCodingKey(stringValue: "city", intValue: nil)],
debugDescription: "How would you describe your relationship with your
debugger?", underlyingError: nil))`
### After
`DecodingError.keyNotFound: Key \'name\' not found in keyed decoding
container. Path: [0]address.city. Debug description: How would you
describe your relationship with your debugger?`
## `test_decodingError_dataCorrupted_emptyCodingPath`
### Before
`dataCorrupted(Swift.DecodingError.Context(codingPath: [],
debugDescription: "The given data was not valid JSON", underlyingError:
Optional(main.GenericError(name: "just some data corruption"))))`
### After
`DecodingError.dataCorrupted: Data was corrupted. Debug description: The
given data was not valid JSON. Underlying error: GenericError(name:
"just some data corruption")`
##
`test_encodingError_invalidValue_nonEmptyCodingPath_nonNilUnderlyingError`
### Before
`invalidValue(456, Swift.EncodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription: "You
cannot do that!", underlyingError: Optional(main.GenericError(name: "You
really cannot do that"))))`
### After
`EncodingError.invalidValue: 456 (Int). Path: first.second[2]. Debug
description: You cannot do that!. Underlying error: GenericError(name:
"You really cannot do that")`
The `CollectionDifference` type has a few different invariants
that were not being validated when initializing using the type's
`Decodable` conformance, since the type was using the
autogenerated `Codable` implementation. This change provides
manual implementations of the `Encodable` and `Decodable`
requirements, and adds tests that validate the failure when trying
to decode invalid JSON for CollectionDifference (and a few other
types).
Restore (un-revert) sting comparison, with fixes
More exhaustive testing of opaque strings, which consistently reproduces prior sporadic failure. Shims fixups. Some test tweaking.
At the cost of slight test readability, improve the diagnostics on test failure by pointing to the specific test case which failed on round-trip failure.
One of the limitations of not having conditional conformance at the
moment is that the implementation of `init(from:)` and `encode(to:)` on
types which require it is that failure to cast dependent types to
`Encodable` or `Decodable` is a runtime failure. There is no way to
statically guarantee that the wrapped type is `Encodable` or
`Decodable`.
As such, in those implementations, at best we can directly call
`(element as! Encodable).encode(to: encoder)`, or similar. However, this
encodes the element directly into an encoder, without giving the encoder
a chance to intercept the type. This is problematic for `JSONEncoder`
because it cannot apply a strategy if it doesn't get to intercept the
type.
This gives a temporary workaround for JSON strategies because of
internal Foundation knowledge.
