This is one of the grayer areas. After your experience writing this decoder ... ...

Stratoscope · on Dec 20, 2011

I've programmed in dozens of languages, and I can't think of another one where loops could automatically run either up or down depending on the start and end values.

Not knowing CoffeeScript well, when I see this code:

n = -10 for i in [0...n] doSomething()

I'd expect it to call doSomething zero times. At least that's what I'd expect in JavaScript or any of the other languages I've used.

jashkenas · on Dec 20, 2011

I agree with you at least 40%, and that's originally how CoffeeScript behaved. Would you still feel that way if you had explicitly written:

    for i in [10..0]

I've opened a ticket to discuss this further: https://github.com/jashkenas/coffee-script/issues/1952

devongovett · on Dec 20, 2011

Hmmm, well I think it's better if they can go in any direction by default otherwise people will be confused. The `by 1` should be considered an optimization IMO.

nitrogen · on Dec 20, 2011

What if you compiled it something like this:

  for i in [a...b]

becomes

  if(a <= b) {
      for(i = a; i < b; i++) {
          // Code
      }
  } else {
      for(i = a; i > b; i--) {
          // Code again
      }
  }

Or otherwise moved the extra conditions outside the loop so they only get computed once?

jashkenas · on Dec 20, 2011

Unfortunately, that requires repeating the entire body of the loop, and so isn't workable/acceptable for our purposes. If you'd like to see the original conversation that led to the current compilation, it's all available on the GitHub issues.

repsilat · on Dec 20, 2011

I'll have a look for the discussion. Any idea if the branches are more expensive than some multiplies?

    step = (a<=b)?1:-1;
    for (i = a; step*i < step*b; i += step) {
      doSomething();
    }

I guess if the branch predictor is any good it could be slower with multiplies, but I have no idea what to expect in a dynamic language.