> On Jul 31, 2017, at 7:23 PM, Daryle Walker via swift-evolution
> <[email protected]> wrote:
>
> When coming up with new array interfaces, I had some array to/from tuple
> conversion functions:
>
> func array<T, U…>(from: (T, …U)) -> [1 + #countOf(U) ; T] allwhere U == T
>
> The problem is going the other way:
>
> func tuple<T, N: Int>(from: [N; T]) -> ???
>
> The tuple will be a bunch of “T,” but the count mechanically added depends on
> compiler-visible number N. Well, since this is part of a proposal that
> already mutates the language, I thought why not add a primitive operation by
> fiat:
>
> func tuple<T, N: Int>(from: [N; T]) -> ( #dup(N; T) )
>
> where “#dup” dumps a comma-separated list repeating the right-side entity
> with a multiplicity of the left-side value. The left-side value has to be a
> compiler constant expression. I was going to have the right side be an
> arbitrary token sequence, but I think it’s better for now to limit it to
> either a type (or similar) or an expression. Obviously, the receiver has to
> compatible with whatever being dumped there.
>
> …
>
> Could we do better than simple repetition? What if we define a “$$n” unit,
> where “n” is a nonnegative integer. For each comma-separated item, the unit
> will be replaced by increasing compiler-constant integers from zero. If using
> an expression, maybe you can call a function on the double-dollar value to
> get your own custom values there. This can be expanded to types once
> value-based generic parameters get added.
>
> When there are nested #dup calls, the double-dollar unit refers to the
> counter for the innermost #dup. Maybe there can be a triple-dollar unit for
> the immediately enclosing #dup. Even quadruple- or more-dollar units for the
> outer levels. (Hopefully, nothing more that one level would be needed.)
>
> …
>
> Later, I remembered the "( 6 * TupleMemberType )” syntax for repeating a type
> within a tuple that's sometimes suggested. The #dup facility can be seen as a
> generalized version of that quasi-array-specific idea.
We've been talking about adding Variadic Generic Parameters for a while, and
I'm pretty sure that the functionality you're suggesting here would be a subset
of that. We're just waiting for it to come into scope.
- Dave Sweeris
_______________________________________________
swift-evolution mailing list
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
