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

David Waite david at alkaline-solutions.com
Wed Apr 6 18:45:00 CDT 2016

> On Apr 6, 2016, at 1:36 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> I think this is a good idea, though I would put the `where` clause after the function signature:
> func foo<T: Foo, U: Bar>(x: T, y: U) -> Result<T,U>
>    where T.Foo == U.Bar /*, etc. */
> {
> }

A bit of a meta-argument:

It is very common to use single-capital letter generic parameters, and the API style does not give guidance around the naming of generic parameters.

However in my humble-but-opinionated view, they are effectively scope-bound, dynamic type aliases. Declaring func "foo<T>" is like declaring “var i”, but its forgiven since coming up with a good, concise name for such a type alias can be hard.

The standard library seems inconsistent about this as well:

func == <A : Equatable, B : Equatable>(_: (A, B), rhs: (A, B))


func == <Key : Equatable, Value : Equatable>(_: [Key : Value], rhs: [Key : Value])

The argument I bring up is that naming of the generic parameters may wind up affecting whether the code is clearer having type constraints and the where clause within the brackets or trailing the function. It is important to take this into account and compare both apples to apples and oranges to oranges when evaluating syntax.

(or, change the API guide and standard library to discourage either apples or oranges)

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