[swift-evolution] [Pre-proposal] Replace [Foo] With CollectionType

Austin Zheng austinzheng at gmail.com
Tue May 24 15:15:24 CDT 2016

Better support for existentials (see the generics manifesto,
should obviate the need for any sort of sugar or compiler magic to do this
kind of thing.

typealias AnyCollection<T> = Any<Collection where .Element == T, ...>
func doSomething(collection: AnyCollection<Foo>)


On Tue, May 24, 2016 at 10:57 AM, Haravikk via swift-evolution <
swift-evolution at swift.org> wrote:

> One thing that I see a lot in code, and sometimes have to stop myself from
> doing, is using shorthand array types, such as [Foo], in function
> declarations where CollectionType could just as easily be used. For
> example, the following two declarations can take collections of values, but
> the first will only take them in the form of an Array:
> func doSomething(values:[Foo]) { … }
> func doSomething<C:CollectionType where C.Generator.Element:Foo>(values:C)
> { … }
> The latter form is something that new users of Swift tend not to know they
> can do, and which even experienced Swift developers may not use for the
> sake of brevity, but it can come up quite a lot. What I’d like to propose
> is that [Foo], when used in a function, should produce the latter form
> behind the scenes, requiring the developer to specify Array<Foo> if they
> actually need it to be an Array for some reason. Though this would become
> inconsistent with variables/properties which would still have to be
> Array<Foo> since a type is required.
> An alternative would be if we could specify protocol generics in a more
> succinct form, for example:
> func doSomething(values:Collection<Foo>) { … }
> func doSomething(values:Sequence<Foo>) { … } // Many array functions are
> linear anyway so could just as easily take sequences
> Note: This would not be the same as type-erased wrappers such as
> AnySequence<Foo>, but rather a shorthand for "Sequence where
> Generator.Element:Foo"
> In essence I’m hoping to discuss whether we should try to remove the
> temptation to limit functions to arrays only or not, or if there are other
> ways to encourage more use of sequence and collection for flexibility; I
> try wherever possible to have my methods take sequences if they can, and
> only take collections if they need to (and never arrays only), but I can
> understand why not everyone does this, as it’s not the friendliest thing to
> add and declaring [Foo] looks so much neater.
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