(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
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