The future of foreach| or just ...

[downs]

Or just do this:

>
> module lockstep;
> import std.stdio;
>
> struct _lockstep(T) {
>   T[] a, b;
>   int opApply(int delegate(size_t id, ref T, ref T) dg) {
>     foreach (id, entry; a) if (auto res=dg(id, entry, b[id])) return res;
>     return 0;
>   }
>   int opApply(int delegate(ref T, ref T) dg) {
>     foreach (id, entry; a) if (auto res=dg(entry, b[id])) return res;
>     return 0;
>   }
> }
>
> _lockstep!(T) lockstep(T)(T[] a, T[] b) { _lockstep!(T) res; res.a=a; res.b=b; return res; }
>
> void main() {
>   foreach (entry1, entry2; lockstep([1, 2, 3], [4, 5, 6]))
>     writefln(entry1, " - ", entry2);
> }
>

:)

 --downs