[swift-evolution] [Review] Constraining AnySequence.init

Kevin Ballard kevin at sb.org
Sat Dec 19 00:03:00 CST 2015

Fair enough. I was thinking that it's better to err on the side of
allowing flexibility, but there is something to be said for having
slices of slices still be slices. My vague thought was that there might
be some desire to have a slice be a type that wraps the thing being
sliced, but I admit that I can't think offhand of any reason why a
FooSlice<FooSlice<FooSlice<Foo>>> would be useful.


On Fri, Dec 18, 2015, at 08:51 PM, Dmitri Gribenko wrote:
> Hi Kevin,
> Thank you for your feedback!
> On Fri, Dec 18, 2015 at 3:13 PM, Kevin Ballard via swift-evolution <swift-
> evolution at swift.org> wrote:
>> 2. One of the added constraints looks like
>> S.SubSequence.SubSequence == S.SubSequence
>> with a comment saying that ideally the set of constraints would apply
>> to the SequenceType protocol but that's not currently possible. This
>> makes sense for the other constraints (that SubSequence conforms to
>> SequenceType and has the same element), but this particular
>> constraint, that the subsequence type must have itself as its own
>> subsequence, surprises me a little. I can see why it's needed here
>> (because that's the only way you can guarantee that recursing through
>> SubSequences always finds SequenceTypes with the right element), but
>> ideally we wouldn't actually require it to be the _same_ sequence,
>> just that it is some sequence with the same element type. If we ever
>> change Swift such that these constraints can be expressed on the
>> SequenceType definition itself, then presumably we'll be able to drop
>> this == constraint entirely as the SequenceType protocol itself will
>> ensure that its subsequence is a sequence of the same element type
>> (which will satisfy the need to have it be true after arbitrary
>> levels of recursion).
> The idea behind putting this constraint into the protocol was to make
> it possible to write code that repeatedly slices a collection without
> getting a ton of different types along the way:
> var myCollectionSlice = myCollection[myCollection.indices]
> myCollectionSlice = myCollectionSlice.dropFirst()
> Also, we couldn't come up with an example of a sequence or a
> collection that needs to have different SubSequence types on different
> depth levels for efficiency or type safety reasons.  Given that we
> think we aren't limiting expressivity with this constraint, having a
> simpler model (the operation of slicing slices is closed in the set of
> types) is better.
> I'd be happy to hear if you know some sequence or collection types
> that would benefit from not having this constraint.
> 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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