[swift-dev] Should we remove _customContainsEquatableElement from stdlib?

Dmitri Gribenko gribozavr at gmail.com
Thu Dec 31 11:21:56 CST 2015

Yes, I think it is good -- this is exactly the use case for this
method, except that @_transparent should be dropped, and the comment
is duplicating the code, so it should be dropped, too.  The change
also needs a simple test that checks that calling .contains() on a
suitable Range should dispatch to this method (you can detect that by
using MinimalComparableValue and supplying it a custom lessImpl that
counts the number of invocations).


On Thu, Dec 31, 2015 at 7:05 PM, Ling Wang <an00na at gmail.com> wrote:
> Following the previous discussion do you think this is a good patch:
> // O(1) implementation of `contains()` for ranges of comparable elements.
> extension Range where Element: Comparable {
>   @_transparent
>   @warn_unused_result
>   func _customContainsEquatableElement(
>     element: Generator.Element
>   ) -> Bool? {
>     return element >= self.startIndex && element < self.endIndex
>   }
> }
>> On Dec 30, 2015, at 6:42 PM, Ling Wang <an00na at gmail.com> wrote:
>> Got it. `contains` is only declared in extension. Interesting workaround. Thanks.
>>> On Dec 30, 2015, at 4:45 PM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
>>> On Wed, Dec 30, 2015 at 8:34 PM, Ling Wang via swift-dev
>>> <swift-dev at 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 <gribozavr at gmail.com>*/

(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/

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