[swift-evolution] Overloading Generic Types

Xiaodi Wu xiaodi.wu at gmail.com
Fri Dec 23 16:02:00 CST 2016

I don't currently have a use for it, but I can certainly see how this might
be useful to some people.

As a side note, though, it seems like the consensus is that the
optimization shown in your specific example, which is provided by
std::vector<bool> in C++, is now widely regarded as a poor design choice.
See, for instance, <
<https://isocpp.org/blog/2012/11/on-vectorbool>, and <

It would be interesting to see if you can come up with a use case where the
proposed feature is a clear win as opposed to just having a totally
separate type that makes clear it's not quite the same thing under the hood
(in your example, something like `BitArray`).

On Fri, Dec 23, 2016 at 3:32 PM, David Sweeris via swift-evolution <
swift-evolution at swift.org> wrote:

> (I feel like I’ve already written this... I looked through my sent mail
> and didn’t see anything, but my sincerest apologies if I started this
> thread a month ago and forgot about it or something.)
> I no longer recall exactly what first made me want to do this (probably
> something in my on-going “teach the compiler calculus” project), but I’ve
> been thinking lately that it could be quite handy to overload types
> themselves based on the value of generic parameters. As a somewhat
> contrived example:
> struct Array <T> { /*everything exactly as it is now*/ }
> struct Array <T> where T == Bool { /* packs every 8 bools into a UInt8
> for more efficient storage */ }
> We can already do this with functions… Conceptually this isn’t any
> different. As long as the specific version exposes everything the generic
> version does (easy for the compiler to enforce), I *think* everything
> would just work (famous last words). In this example, the subscript
> function would need to extract the specified bit and return it as a Bool
> instead of simply returning the specified element. The `Element` typealias
> would be `Bool` instead of `UInt8`, which would mean the size/stride might
> be different than expected (but that’s why we have `MemoryLayout<>`).
> Anyway, because generic structs & functions already can’t make assumptions
> about a generic argument (beyond any constraints, of course), I *think*
> this should be in phase 2… but I’m quite hazy on how generics work once the
> code’s been compiled, so maybe not.
> - Dave Sweeris
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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