[swift-evolution] [Proposal] More Powerful Constraints for Associated Types

David Hart david at hartbit.com
Fri Apr 29 03:50:31 CDT 2016

Hi Dimitri,

Excuse me if I’m being dumb. I saw those example but I don’t understand their use.

If I define a custom Collection, I still have the be explicit in the types I use where associated types are expected. Because I need to be explicit, I don’t understand how default associated types are used.

Default parameters seem clear to me:

func foobar(a: Int = 0) {}
foo() // implicit uses default value
foo(a: 1) // Explicit bypasses default value

But default associated types are not:

protocol Foobar {
    associatedtype Foo = Int
    func foobar(a: Foo)

class FoobarImpl : FooBar {
    func foobar(a: Foo) {} // error: Use of undeclared type ‘Foo’ - there is no such thing as an implicit use
    func foobar(a: Double) {} // This works but is explicit

I just don’t see the implicit use of the default associated type.


> On 29 Apr 2016, at 09:36, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> On Fri, Apr 29, 2016 at 12:25 AM, David Hart via swift-evolution
> <swift-evolution at swift.org> wrote:
>> I’ve taken some time to digest the current feedback and I’ve changed my mind. The syntax for adding constraints to a sub-protocol in the protocol’s definition where clause is starting to grow on me. Before I modify the proposal, I'd still like to understand something:
>> What is the use of declaring a default associated types with the `=` syntax in protocols? I’ve never used them and I don’t understand what they provide.
> Please take a look at the Collection protocol
> (https://github.com/apple/swift/blob/master/stdlib/public/core/Collection.swift).
> associatedtype Iterator : IteratorProtocol = IndexingIterator<Self>
> associatedtype SubSequence : IndexableBase, Sequence = Slice<Self>
> associatedtype Indices : IndexableBase, Sequence = DefaultIndices<Self>
> Some collections want to customize these, but for simple ones the
> defaults are usually fine.
> Dmitri
> -- 
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/

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