Swift: Protocol conformance does not distribute over tuples
| Originator: | rix.rob | ||
| Number: | rdar://17478368 | Date Originated: | 26-Jun-2014 09:53 PM |
| Status: | Open | Resolved: | |
| Product: | Developer Tools | Product Version: | Xcode6-Beta2 (6A216f) |
| Classification: | Serious Bug | Reproducible: | Always |
Summary:
The tuple type (A, A) where A is e.g. Equatable is not itself Equatable. Ditto Hashable.
Likewise for the tuple type (A, B) where A : Hashable, B : Hashable.
I don’t know that this would apply sensibly to *all* protocols (I think it would apply to Collection for example—that’s zip2!—but IntegerLiteralConvertible…?), but Equality, Hashable, and Printable sure seem doable. Unfortunately, they are not doable not by third parties.
Steps to Reproduce:
1. Try to use that sweet memoize { … } combinator from session 404 Advanced Swift to make a function whose parameter type is a tuple of (A, A), where A : Hashable.
Expected Results:
I expected it to succeed.
Actual Results:
It does not—and the error is impossibly opaque.
Regression:
N/A
Notes:
Since you can’t extend tuples, this means you can’t use tuples for any number of valuable tasks.
Comments
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