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More Powerful Constraints for Associated Types (#284)
* formalized proposal already discussed * added more details pertaining to constraints on parent associated types * added examples to name lookup * nitpick * Rename for more clarity * Improvements after comments from @dabrahams * Updated links to mailinglist instead of gmane * Updated header format and email links * escaped the `where` keyword * Minor corrections suggested by @dabrahams * Added paragraph on ABI stability
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# Permit where clauses to constrain associated types | ||
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* Proposal: [SE-NNNN](NNNN-associated-types-constraints.md) | ||
* Authors: [David Hart](https://github.com/hartbit), [Jacob Bandes-Storch](https://github.com/jtbandes), [Doug Gregor](https://github.com/DougGregor) | ||
* Review Manager: TBD | ||
* Status: **Awaiting review** | ||
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## Introduction | ||
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This proposal seeks to introduce a `where` clause to associated type | ||
declarations and improvements to protocol constraints to bring associated types | ||
the same expressive power as generic type parameters. | ||
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This proposal was discussed twice on the Swift Evolution list in the following | ||
threads: | ||
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* [\[Completing Generics\] Arbitrary requirements in protocols](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160411/014667.html) | ||
* [Proposal] More Powerful Constraints for Associated Types | ||
- [Week #1](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160418/015625.html) | ||
- [Week #2](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160425/015753.html) | ||
- [Week #3](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160502/016354.html) | ||
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## Motivation | ||
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Currently, associated type declarations can only express simple inheritance | ||
constraints and not the more sophisticated constraints available to generic | ||
types with the `where` clause. Some designs, including many in the Standard | ||
Library, require more powerful constraints for associated types to be truly | ||
elegant. For example, the `SequenceType` protocol could be declared as follows | ||
if the current proposal was accepted: | ||
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```swift | ||
protocol Sequence { | ||
associatedtype Iterator : IteratorProtocol | ||
associatedtype SubSequence : Sequence where SubSequence.Iterator.Element == Iterator.Element | ||
... | ||
} | ||
``` | ||
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## Detailed Design | ||
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First of all, this proposal modifies the grammar for a protocol's associated types | ||
to the following: | ||
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*protocol-associated-type-declaration* → | ||
*attributes<sub>opt</sub>* | ||
*access-level-modifier<sub>opt</sub>* | ||
**associatedtype** | ||
*typealias-name* | ||
*type-inheritance-clause<sub>opt</sub>* | ||
*typealias-assignment<sub>opt</sub>* | ||
*requirement-clause<sub>opt</sub>* | ||
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The new requirement-clause is then used by the compiler to validate the | ||
associated types of conforming types. | ||
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Secondly, the proposal also allows protocols to use the associated types of | ||
their conforming protocols in their declaration `where` clause as below: | ||
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```swift | ||
protocol IntSequence : Sequence where Iterator.Element == Int { | ||
... | ||
} | ||
``` | ||
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Name lookup semantics in the protocol declaration `where` clause only looks at | ||
associated types in the parent protocols. For example, the following code would | ||
cause an error: | ||
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```swift | ||
protocol SomeSequence : Sequence where Counter : SomeProtocol { // error: Use of undefined associated type 'Counter' | ||
associatedtype Counter | ||
} | ||
``` | ||
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But instead should be written on the associated type itself: | ||
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```swift | ||
protocol IntSequence : Sequence { | ||
associatedtype Counter : SomeProtocol | ||
} | ||
``` | ||
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## Effect on ABI Stability | ||
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As mentioned previously, there are a number of places in the standard library where this feature would be adopted (such as the `SubSequence.Iterator.Element == Iterator.Element` example), each of which will change the mangling of any generic function/type that makes use of them. | ||
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## Alternatives | ||
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Douglas Gregor argues that the proposed syntax is redundant when adding new | ||
constraints to an associated type declared in a parent protocol and proposes | ||
another syntax: | ||
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```swift | ||
protocol Collection : Sequence { | ||
where SubSequence : Collection | ||
} | ||
``` | ||
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But as Douglas notes himself, that syntax is ambiguous since we adopted the | ||
generic `where` clause at the end of declarations of the following proposal: | ||
[SE-0081: Move where clause to end of declaration](https://github.com/apple/swift-evolution/blob/master/proposals/0081-move-where-expression.md). For those reasons, it might be wiser not to introduce the shorthand syntax. | ||
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## Acknowledgements | ||
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Thanks to Dave Abrahams and Douglas Gregor for taking the time to help me | ||
through this proposal. |