I'm looking for consensus on, and a coalition for, including generic subscripts
in Phase 1 of the Swift 4 design cycle.
The Need
-------------
While prototyping my deferred [SE-0132][], I ran into trouble introducing
`RangeExpression`, an abstraction to make the introduction of subrange features
easier. Since RangeExpression is a protocol compatible with any index type, it
has to have an associated type, and thus cannot be handled with an existential.
I therefore had to add an overloaded subscript for each type, which partially
defeated the purpose of having the protocol.
The lack of generic subscripts also forced a nasty workaround for a convenience
subscript in [SE-0131][]. That proposal extends `Dictionary where Key ==
AnyHashable` (actually a hidden `_AnyHashableProtocol`, but that's a different
story) with a convenience subscript which takes any `Hashable` type—except
that, because generic subscripts are impossible, [it instead has to take a
hidden `_Hashable` type instead][anyhash-subscript].
The generics manifesto suggests a third use case: [a subscript that can take
any Collection of Index][manifesto].
The lack of this feature at the very least impacts binary compatibility
directly. It also affects source compatibility in that features like subranges
are designed around its absence, forcing workarounds which affect userspace. I
see good reasons to do it now and few to delay.
Prior Art
-----------
As mentioned, SE-0131 and SE-0132 would have benefited from this feature.
After a brief and mostly positive [discussion][], Harlan Haskins and Robert
Widmann submitted a [pull request][] late in the Swift 3 cycle for generic and
throwing subscripts. Personally, I think throwing subscripts are something we
should include, but they're a separate issue and may not be appropriate for
Phase 1; I would suggest severing the two parts of the proposal.
Next Steps
---------------
What I'd like to figure out at this point is:
* Who is interested in this feature?
* Should it be severed from throwing subscripts?
* Does the core team agree that this is in Phase 1's scope?
* Do the people who might be able to implement this have any comments on it?
[SE-0132]:
<https://github.com/apple/swift-evolution/blob/master/proposals/0132-sequence-end-ops.md>
[SE-0131]:
<https://github.com/apple/swift-evolution/blob/master/proposals/0131-anyhashable.md#detailed-design>
[anyhash-subscript]:
<https://github.com/apple/swift/blob/e051c61c4d7eb33cdbb47b8ac04eae38203a61e6/stdlib/public/core/HashedCollectionsAnyHashableExtensions.swift.gyb#L147>
[manifesto]:
<https://github.com/apple/swift/blob/e3d8448bbdd059a55a6e72c24d07e994afaf5926/docs/GenericsManifesto.md#generic-subscripts>
[discussion]:
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/021450.html>
[pull request]: <https://github.com/apple/swift-evolution/pull/372>
--
Brent Royal-Gordon
Architechies
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