[swift-evolution] [Pitch] Moving where Clauses Out Of Parameter Lists

[Chris Lattner]

> On Apr 6, 2016, at 11:30 AM, Developer via swift-evolution <swift-evolution at swift.org> wrote:
> 
> If you've ever gotten to the point where you have a sufficiently generic interface to a thing and you need to constrain it, possibly in an extension, maybe for a generic free function or operator, you know what a pain the syntax can be for these kinds of operations.  For example, the Swift book implements this example to motivate where clauses
> 
> func anyCommonElements <T: SequenceType, U: SequenceType where T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element> (lhs: T, _ rhs: U) -> Bool
> 
> This is noisy and uncomfortable to my eyes, and almost impossible to align correctly.  Per a short discussion on Twitter with Joe Groff and Erica Sadun, I'd like so see what the community feels about moving the where clause out of the angle brackets.  So that example becomes
> 
> func anyCommonElements <T: SequenceType, U: SequenceType>
> where T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element
> (lhs: T, _ rhs: U) -> Bool
> 
> Or, if you're feeling ambitious, even
> 
> func anyCommonElements <T, U>
> where T : SequenceType, U : SequenceType,
> T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element
> (lhs: T, _ rhs: U) -> Bool
> 
> Thoughts?

+1, long overdue.  Please keep basic constraints (ones expressible without a ‘where’ clause, like simple conformances) inline though.

-Chris