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

Ling Wang an00na at gmail.com
Thu Dec 31 11:05:43 CST 2015

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 {
  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>*/

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