Prettier iterator implementations in D?

[Reiner Pope]

The proposal looks good (as long as it creates no syntactical 
ambiguities) but if we had variadic template parameters (see 
http://www.generic-programming.org/~dgregor/cpp/variadic-templates.html 
for a good version for C++) then you could do it much more easily with 
templates:

template IterFunc(type ...)
{
     alias int delegate(type ...) IterFunc;
}

and then you could just declare your iterators:

int opApply(IterFunc!(int) dg)

The best thing about this is you can easily modify it:

template IndexedIterFunc(type ...)
{
     alias int delegate(size_t index, type ...) IndexedIterFunc;
}

or if the return status was instead the first function parameter (to 
allow for return values, not void):

template NewIterFunc(retVal, type ...)
{
   alias retVal delegate(out int status, type ...) NewIterFunc;
}

Unfortunately, this wouldn't handle 'inout' parameters, since you can't 
instantiate a template with an inout parameter...

Cheers,

Reiner