[swift-evolution] [proposal] Generic type aliases

Russ Bishop xenadu at gmail.com
Wed Mar 16 14:23:52 CDT 2016

> On Mar 9, 2016, at 8:47 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> This is a minimal proposal for introducing type aliases into Swift, and intentionally chooses to keep them limited to being “aliases”.  As such, additional constraints are not allowed in this base proposal, e.g. you can’t write:
>     typealias StringDictionary<T where T : Hashable> = Dictionary<String, T>

What is the rationale for not supporting constraints? Generic type aliases have value without constraints but I can also see a lot of uses for allowing them, especially in some cases where the type constraints are fairly complex and repetitive. It also serves the same self-documenting-code purposes.

In this example it would be nonsense to have a “MatcherFunction” that can’t compare values. Why not express that in the constraint?

    typealias MatcherFunction<T where T : Comparable> = (T, T) -> MatchResult


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