> 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.
--
Brent Royal-Gordon
Architechies
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