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Comment by jbanon | Hacker News Reader

This is the difference in compiled output for those examples:

  var i;
  
  for (i = 0; i < 10; i++) {
    doSomething();
  }
  
  for (i = 0; i < 10; i += 1) {
    doSomething();
  }

http://jashkenas.github.com/coffee-script/#try:for%20i%20in%...

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Oops, I should have written it using a variable instead... i.e.

  for i in [0...test]
    doSomething()

  for i in [0...test] by 1
    doSomething()

compiles to:

  var i;

  for (i = 0; 0 <= test ? i < test : i > test; 0 <= test ? i++ : i--) {
    doSomething();
  }

  for (i = 0; i < test; i += 1) {
    doSomething();
  }

the latter one being much faster because it isn't testing the direction the counter should be going all the time.

This is one of the grayer areas. After your experience writing this decoder ... would you prefer CoffeeScript to keep its loops that can go in either direction, or would you prefer loops to always iterate upwards, and have to be explicit if you'd like to count from "[100..1] by -1" ?

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.

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?

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.

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.

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.

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

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