Multiple generators in list comprehensions

[gampleman]

I was writing a Haskell tutorial for my students today. They were to write a function that returns all the possible sums of two lists:

sums as bs = [a + b | a <- as, b <- bs]

So I thought: let's try that in coffee:

sums = (as, bs) -> a + b for a in as for b in bs

which produces this beast of js:

var sums;

sums = function(as, bs) {
  var a, b, _i, _len, _results;
  _results = [];
  for (_i = 0, _len = bs.length; _i < _len; _i++) {
    b = bs[_i];
    _results.push((function() {
      var _j, _len1, _results1;
      _results1 = [];
      for (_j = 0, _len1 = as.length; _j < _len1; _j++) {
        a = as[_j];
        _results1.push(a + b);
      }
      return _results1;
    })());
  }
  return _results;
};

As you can see this will produce an array of arrays rather then a single array. Another problem comes when you wish to use guards:

sums = (as, bs) -> a + b for a in as for b in bs when a > b

This will not work, as a is not in scope yet.

---

My proposal is to change the semantics to allow guard expressions at the end of the list comprehension to be evaluated in the inmost loop.

Also the loop should be compiled rather to this:

var sums;

sums = function(as, bs) {
  var a, b, _i, _len, _results;
  _results = [];
  for (_i = 0, _len = bs.length; _i < _len; _i++) {
    b = bs[_i];
    var _j, _len1;
    for (_j = 0, _len1 = as.length; _j < _len1; _j++) {
      a = as[_j];
      _results.push(a + b);
    }
  }
  return _results;
};

[vendethiel]

Discussed multiple times already #2030
Also see http://brehaut.net/blog/2011/coffeescript_comprehensions

[gampleman]

Ah the language I used was a bit different, so I couldn't find it. Thanks. Closing for now.

[vendethiel]

That's my job ;)

[michaelficarra]

FWIW, I'd prefer to switch to the proposed semantics.

[jashkenas]

Yes -- I think it's worth talking about (over in #2030), especially if we can have our cake and eat it too -- have a way of allowing the inner comprehension to either be treated as an expression, or flattened out.