Proposal: delegates as aggregates in foreach statements

[Chris Nicholson-Sauls]

An idea occurred to me last night, which I'm sure must have come up before.  If it hasn't, 
I'm shocked, but I'm bringing it up (again?) anyway.  Why not allow a delegate (or even 
function pointer?) to be used as the "aggregate" parameter to a foreach statement, 
requiring that it expose the same signature as a valid opApply method?  For example:

# class Foo {
#   private int[] p_data;
#
#   int opApply (int delegate(inout size_t, inout int) dg) {
#     int result = 0;
#     foreach (inout i, inout x; p_data) {
#       result = dg(i, x);
#       if (result)
#         break;
#     }
#     return result;
#   }
#
#   int reverse (int delegate(inout size_t, inout int) dg) {
#     int result = 0;
#     foreach (inout i, inout x; p_data.reverse) {
#       result = dg(i, x);
#       if (result)
#         break;
#     }
#     return result;
#   }
# }
#
# Foo foo = new Foo;
# foreach (size_t i, int x; &foo.reverse)
#   // ... do stuff ...

One could even get real cute and use anonymous delegates:

# foreach (size_t i, inout char[] x;
#   delegate int(int delegate (inout size_t ii, inout char[] xx) {
#     // implement iterator logic
#   }
# ) {
#   // ... do stuff ...
# }

This, I think, would stand in the place of many uses of iterator objects and mutators.

-- Chris Nicholson-Sauls