Hello all,
Currently there exist at least four 'type-erased' sequence/collection types: AnySequence, AnyForwardCollection, AnyBidirectionalCollection, and AnyRandomAccessCollection.
The three Any*Collection types can be conceptually arranged into a 'ladder', in which collections can be constructed unconditionally from collection types above themselves, and conditionally from collection types below themselves. I've put together a little ASCII image of this here: (https://gist.github.com/austinzheng/829425242bef1573b668\).
Note that AnySequence doesn't currently fit into this ladder: Any*Collection instances can't easily be constructed from AnySequences. SR-119 (https://bugs.swift.org/browse/SR-119\) aims to change that. As per that ticket, I propose that we add three promotion APIs, sketched out as following:
extension AnyForwardCollection {
init?<Element>(_: AnySequence< Element>)
}
extension AnyBidirectionalCollection {
init?<Element>(_: AnySequence< Element>)
}
extension AnyRandomAccessCollection {
init?<Element>(_: AnySequence< Element>)
}
Like their Any*Collection --> Any*Collection antecedents, these initializers construct a new Any*Collection out of an existing AnySequence, but only if the underlying sequence is compatible, and without copying the underlying sequence.
I'll also have to examine SE-0014 (https://github.com/apple/swift-evolution/blob/master/proposals/0014-constrained-AnySequence.md\) more closely to see if it has any effects on this proposal.
I plan on submitting a formal proposal at some point (probably by the end of the coming week), but first wish to solicit feedback, opinions, concerns, etc. Thanks for your time.
Best,
Austin