template emit(T, alias fun)
I wonder if "template emit(alias fun, T)" wouldn't be a better
idea. That way you could leave T open for type inference, even if
you specify a function. D doesn't support reordering template
arguments, so it's important to put the ones "most used" at the
beginning.
Anyway, it looks neat. I hope you've looked at
std.algorithm.iteration.reduce though? It does the same thing
that emit does, and it's an ancient algorithm.
"fold" is just reduce with the arguments more convenient for D's
syntax. But...
assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a);}).equal([1, 9,
25, 49]));
=>
int[] intderp;
assert(9.iota.fold!((result,a) { if(a%2) return result ~ [a*a];
else return result;})(intderp) == ([1,9,25,49]));
What would be neat is a "reduce" that could produce an infinite
range as a result. All I can do with reduce and fold is produce
an eagerly evaluated growing array in memory, or something like
chain(chain(chain(chain(chain(...))))) which uses up memory just
from creating all those iterators. Like, I don't know how to use
fold! to do this:
auto natural_numbers = sequence!"n"();
auto oddsquared = natural_numbers.filter!"a%2==1".map!"a*a"; //
fold?
assert(take(oddsquared,4).equal([1, 9, 25, 49]));
