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

Leonardo Pessoa me at lmpessoa.com
Tue May 24 15:14:39 CDT 2016

My first question here would be "what's the gain?" I understand
CollectionTypes and arrays may be something different but I myself
admit I don't know and wonder how that change would be a benefit to

On 24 May 2016 at 14:57, 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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