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

[Joe Groff]

> On Apr 6, 2016, at 12:52 PM, Pyry Jahkola <pyry.jahkola at iki.fi> wrote:
> 
> Joe,
> 
> Just from your experience on this topic, is there any reason not to also move the primary constraints into the trailing `where` clause?
> 
> So instead of what you wrote, we'd have it this way:
> 
>     func foo<T, U>(x: T, y: U) -> Result<T,U>
>        where T: Foo, U: Bar, T.Foo == U.Bar /*, etc. */
>     {
>     }
> 
> …as well as:
> 
>     struct Foo<T, U>
>        where T: Foo, U: Bar, T.Foo == U.Bar
>     {
>     }
> 
> Like I said earlier in this thread, I think this would also make the `extension` syntax more uniform with types (by turning generic parameters into strictly locally visible things):
> 
>     extension Foo<T, U> where U == Baz { // (Could've used X and Y here as well.)
>         // Now it's clear where the names T and U come from.
>         var bazzes: [U] { return ... }
>     }

It's a judgment call. It's my feeling that in many cases, a generic parameter is constrained by at least one important protocol or base class that's worth calling out up front, so it's reasonable to allow things like 'func foo<C: Collection>(x: C) -> C.Element' without banishing the 'Collection' constraint too far from the front of the declaration.

-Joe