DIP58: ".." as a Binary Operator

[Mason McGill]

> Random thought, but treating step as a template argument would 
> allow for some more interesting changes to the iterations, 
> though I can't think of any particular syntax that would look 
> good. And if you did add such a thing, then other operators 
> would want it as well.

Interesting, though I feel like operator syntax really shines 
when either
   1) It's significantly shorter/simpler than the equivalent 
function calls.
   2) It appeals to domain-specific intuitions.

In terms of (1) and (2), I think better spellings for these might 
be

> int[] foo = a ..!"a += 10" b;

import std.array: array;
import std.range: iota;

int[] foo = array(iota(a, b, 10));
// Or something with recurrence, in the general case.

> bool equals = dbl1 ==."abs(a - b) < 0.01" dbl2

import std.math: abs;

struct Approximation(A)
{
     A a;
     alias a this;

     bool opEquals(B)(B b) const
       { return abs(a - b) < 0.01; }
}

auto approximate(A)(A a)
   { return Approximation!A(a); }

bool equals = approximate(a) == b;