Return type of std.algorithm.map

[James Fisher]

To date, I've been using D in much the same way I used C++, without heavy
use of templates.  Now I'm trying out a more functional style using
std.algorithm.  However I'm pretty much stuck at the first hurdle: map.
 With type inference, this works:

import std.algorithm;
import std.stdio;

void main() {
  auto start = [1,2,3,4,5];
  auto squares = map!((a) { return a * a; })(start);
  writeln(squares);
}


Without type inference (obviously necessary beyond trivial examples), it'd
be nice to do:

import std.algorithm;
import std.stdio;

void main() {
  int[] start = [1,2,3,4,5];
  int[] squares = map!((a) { return a * a; })(start);
  writeln(squares);
}


but this gives "Error: cannot implicitly convert expression (map(start)) of
type Result to int[]".  That opaque type "Result" is weird (not
parameterized on "int"?), but OK, let's try that:

import std.algorithm;
import std.stdio;

void main() {
  int[] start = [1,2,3,4,5];
  Result squares = map!((a) { return a * a; })(start);
  writeln(squares);
}


gives "undefined identifier Result".  Not sure why, but OK.  I can't see
examples in the docs of explicit type declarations -- annoyingly they all
use "auto".

However, they do tell me that map "returns a range".  Assuming all
definitions of ranges are in std.range, there's no such thing as a "Range"
interface, so it's not that.  The most general interfaces I can see are
InputRange(E) and OutputRange(E).  It certainly can't be an OutputRange, so
I guess it must satisfy InputRange, presumably type-parameterized with
"int".  So this should work:

import std.algorithm;
import std.stdio;
import std.range;

void main() {
  int[] start = [1,2,3,4,5];
  InputRange!int squares = map!((a) { return a * a; })(start);
  writeln(squares);
}


But the compiler complains "cannot implicitly convert expression
(map(start)) of type Result to std.range.InputRange!(int).InputRange".
 That's weird, because "std.range.InputRange!(int).InputRange" doesn't even
look like a type.

I've tried all manner of combinations here, but nothing works.  Seems like
there's something fundamental I'm not understanding.
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