On Wed, Dec 30, 2015 at 8:34 PM, Ling Wang via swift-dev
<swift-dev@swift.org> wrote:
> After reviewing the code of stdlib I found no one actually implements
> _customContainsEquatableElement:
> 1. Its default implementation in `SequenceType` just returns nil.
> 2. The implementation in `Set` delegates to `contains` which is bad because
> it reverses their relationship: the default implementation of `contains` in
> `SequenceType` delegates to `_customContainsEquatableElement`.
> 3. In all other place it just delegates to another `SequenceType`.
>
> So no one is doing real work.
>
> If the current _customContainsEquatableElement is only a relic I suggest we
> remove it and clean up related code.
It is not a relic. It allows the contains() method to perform dynamic
dispatch in case the container knows something extra about the
elements.
Consider the case when you have a Set typed as a plain Sequence or a
Collection. In that case, you won't be able to call the custom
Set.contains() method, the overload resolution will only see the
protocol extension. This extra entry point,
_customContainsEquatableElement, allows us to perform dynamic dispatch
and use the fast Set implementation if it is available.
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 <griboz...@gmail.com>*/
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