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

Matthew Johnson matthew at anandabits.com
Fri May 27 13:47:59 CDT 2016

> On May 27, 2016, at 12:05 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
>> On May 27, 2016, at 9:31 AM, plx via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> For the Sequence/Collection it’s a lot of work for IMHO a rather minor convenience, but for more-complex type associated-type relationships it could start to pay its own way.
> Is it really that minor, though? For something so commonly encountered as methods that take sequences/collections, this:
> func doSomething(foos: [Foo], bars: [Bar], bazzes: [Baz])
> is not only a whole lot easier to type, but is worlds clearer to read than:
> func doSomething<S: SequenceType, T: SequenceType, U: SequenceType where S.Generator.Element == Foo, T.Generator.Element == Bar, U.Generator.Element == Baz>(foos: S, bars: T, bazzes: U)
> Not only is the latter line intimidating to look at as is, but it separates the contained type from the parameters themselves. Given how unwieldy this second form is, it seems almost certain that the former line will be used more frequently in the real world.

When generalized existentials are introduced (Austin Zheng has a proposal for this) you will be able to do this (assuming we switch to the `&` syntax:

typealias SequenceOf<T> = Sequence where .Element == T

func doSomething(foos: SequenceOf<Foo>, bars: SequenceOf<Bar>, bazzes: SequenceOf<Baz>)

It’s still slightly more verbose than the array shorthand, but it’s a far cry from what you have to do with generics today.

If you wanted it to be generic you could write it as:

func doSomething<S: SequenceOf<Foo>, T: SequenceOf<Bar>, SequenceOf<Baz>(foos: S, bars: T, bazzes: U)

> Charles
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