> On Dec 4, 2017, at 18:48, Brent Royal-Gordon via swift-dev
> <swift-dev@swift.org> wrote:
>
>> On Dec 2, 2017, at 12:31 PM, Cao, Jiannan via swift-dev
>> <swift-dev@swift.org> wrote:
>>
>> I'd like to discuss the relation between RangeReplaceableCollection and
>> MutableCollection
>>
>> MutableCollection is a Collection type that can { set } any element with
>> subscript(position: Index). (and also { set } for subscript(range:
>> Range<Index>))
>>
>> RangeReplaceableCollection requires a function replaceRange(_:with:)
>>
>> If a type conforms to RangeReplaceableCollection, it means any of its range
>> can be replaced, that includes the situation to replace only one element.
>> So if some type conforms to RangeReplaceableCollection, it is sufficient to
>> be a MutableCollection.
>> So I think the RangeReplaceableCollection should conforms to
>> MutableCollection should inherits from MutableCollection.
>
>
> I thought this too a couple years ago, but it's not really true.
> `MutableCollection` requires not only that you be able to set elements, but
> also that when you do so, it doesn't change any of the indices in the
> collection. Some collections can't guarantee that. For example, `String`
> can't conform to `MutableCollection` because if you set some character in the
> middle of the string to a character that's a different size, it might move
> characters later in the string to different indices. So Swift lets you say
> `myString.replaceSubrange(myRange, with: newCharacters)`, but it doesn't let
> you say `myString[myRange] = newCharacters`.
>
> `MutableCollection` and `RangeReplaceableCollection` are very similar in that
> they both involve changing a collection. But they are different *kinds* of
> changes to a collection, so it makes sense for a collection to support either
> one of them without supporting the other.
MutableCollection's subscript setter is also expected to take constant time
(well, proportional to the size of a single element). That may not be the case
for a range replacement implementation, even if it turns out the other elements
in the collection don't have to move.
Jordan
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