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

[Jordan Rose]

> On Apr 6, 2016, at 16:52, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Apr 6, 2016, at 5:45 PM, David Waite via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Apr 6, 2016, at 1:36 PM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto: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))
>> 
>> vs.
>> 
>> 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)
>> 
>> -DW
> 
> I'll keep this short. IMO:
> 
> * Dictionaries have semantics, so Key/Value makes sense.
> * Truly "generic" equatable values do not, so A and B are simple stand-ins.
> * Always prefer named tokens when there are actual semantics (Element, Wrapped, etc). 
> 
> This may or may not be appropriate for inclusion in the API guidelines.

Another factor here is that we've been planning for a while for generic parameters to types to be exposed as implicit member typealiases, since they already conflict with the namespace for member typealiases. Therefore it's important to name generic parameters to types well, but less important to do so for generic parameters for functions. (They're also much less likely to be ad hoc for types; there has to be something that describes the relation between the Self type and the parameter, while the function might not have anything more interesting than "operand".)

This is also a minor point against declaring generic parameters for extensions, since they would have to match up. We also do need a syntax some day to represent new parameters:

// For expository purposes only:
extension <T> Array where Element == Optional<T>

But that deserves its own proposal. I just don't want us to paint ourselves into a corner.

Jordan

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