[swift-evolution] [swift-evolution-announce] [Review] SE-0048: Generic Type Aliases

Chris Lattner clattner at apple.com
Mon Mar 28 23:28:21 CDT 2016

On Mar 24, 2016, at 1:18 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> Is the problem being addressed significant enough to warrant a change
>> to Swift?
> That's my biggest question about this proposal.  Yes, as the proposal
> says, generic type aliases fill an obvious functional gap.  But what
> real-world problems are they solving

They allow you to make aliases for generic types, e.g. from the document:

	typealias StringDictionary<T> = Dictionary<String, T>
	typealias DictionaryOfStrings<T : Hashable> = Dictionary<T, String>
	typealias IntFunction<T> = (T) -> Int
	typealias Vec3<T> = (T, T, T)
	typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1)

It is an inconsistency in the language that you can create aliases for non-generic types, but that you can’t make them for generic ones.  Perhaps you are objecting to typealias as a feature at all?

> and why should adding them be a priority?

It is part of the general goal of completing the generics system, a stated Swift 3 goal.


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