> On Dec 8, 2016, at 4:35 PM, Jordan Rose via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Um, Sequence doesn’t have a subscript (or indexes). Sequences are
> single-pass. So if this is important, it needs to stay a Collection.
>
Just because something fulfills one of the requirements of a Collection does
not mean it should be one. It needs to tick all the boxes before its allowed to
be elevated.
But it’s still allowed to have subscripts (UnsafePointer has subscripting but
isn’t a collection) or be multi-pass (strides are multiples but are only
sequences). That’s OK
In this case, yes it’s multi-pass, yes it has a subscript, but no it isn’t a
collection because it doesn’t meet the requirements for slicing i.e. that
indices of the slice be indices of the parent.
(relatedly… it appears this requirement is documented on the concrete Slice
type rather than on Collection… which is a documentation bug we should fix).
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