Variadic Generics

Evolution Pitches
98

I thought about variadic generics recently because it was anounced that Swift 6 will have this function at here.

I found this thread and read draft of proposal.
Then I want to improve expression of curry.

One existing idea is this.

func curry<A, B, C, variadic D, Result>(_ fn: @escaping (A, B, C, D...) -> Result) -> ???

It seems to infer type of ???.
But it is not appropriate for Swift which always describes signature of function.

Another is this.

enum CurryHelper<variadic T, Result> {
#ifempty T
  typealias Fn = Result
#else
  typealias Fn = (#head(T)) -> CurryHelper<#tail(T), Result: Result>.Fn
#endif
}

I think its good for point of that it is succeeded to express.
But rule of evaluating #ifempty happens when application of generic parameters is not consistent with existing evaluation of #if.
#if is evaluated statically only once if it is used in declaration like enum.
This is confusing behavior for programmer.

So I propose #condtype as a new operation to represent branching in compile time.
It can be expressed as below with this.

enum CurryHelper<vairadic T, Result> {
  typealias Fn = #condtype(#ifempty(T), 
    Result,
    (#head(T)) -> CurryHelper<#tail(T), Result>.Fn)
}

Or we can eliminate namespace by CurryHelper.

typealias CurryFn<variadic T, Result> = #condtype(
    #ifempty(T), 
    Result,
    (#head(T)) -> CurryFn<#tail(T), Result>
)
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