Tuple DIP

[Timon Gehr]

On 13.01.2018 01:20, Mark wrote:
> 
> Could we also support function tuples?

In principle, yes, though I imagine it is a lot harder to argue for its 
inclusion than for that of the features currently proposed in the DIP, 
because existing language features already come rather close.

> For instance:
> 
> int sum(int a, int b) { return a+b; }
> int diff(int a, int b) { return a-b; }
> 
> (int, int) f = (sum, diff) // no ";" for consistency with existing syntax

No ";" is inconsistent. Also the type of f cannot be (int, int), and 
(sum, diff) would try to call sum and diff with 0 arguments and fail.

> int (x,y) = f(1, 2); // x=3, y=-1
> int (w,z) = (1, 2).f() // same as above, UFCS
> int (u,v) = (1, 2).(sum, diff) // same as above, "lambda tuple"
> 
> In the last example, (sum, diff) is basically lowered to (x,y) => 
> (sum(x,y), diff(x,y)).

I think this is easy enough to achieve without dedicated syntax:

import std.typecons;

template apply(F...){
     auto apply(T)(T arg){
         import std.conv, std.range, std.algorithm;
         return 
mixin(text("(",iota(F.length).map!(i=>text("F[",i,"](arg)")).join(","),")"));
     }
}

void main(){
     int sum(int a, int b) { return a+b; }
     int diff(int a, int b) { return a-b; }
     auto (u,v)=(1,2).apply!(sum, diff);
     import std.stdio;
     writeln(u," ",v);
}