Comprehensions can't handle arguments objects.

[jashkenas]

A little help would be good on this one. Our array/object comprehensions can't handle argument objects being passed. The for..in loop doesn't see any properties on them, and doesn't run. Is there any way around this, other than generating two code paths for comprehensions (one for arrays/arguments and one for objects)? See the following JS:

for (key in [1, 2, 3]) console.log(key)
=> 0
=> 1
=> 2
=> undefined

(function(){ for (key in arguments) console.log(key) })(1, 2, 3)
=> undefined

[weepy]

could CS replace the arguments keyword with an Array.spliced version ? I thought about suggesting this before.

x: => 
  for arg in arguments
    ...

var x
x = function x() {
 var _arguments = Array.prototype.slice.call(arguments,0,arguments.length)
 for(var i in _arguments) 
    ....

(function(){ var args = Array.prototype.slice.call(arguments,0,arguments.length); for (key in args) console.log(key) })(1, 2, 3) // => 1 2 3

[jashkenas]

We could add it as a check on obj.length -- you don't want to do it for vanilla objects (it makes them disappear), just arguments objects. So then you'd have to do the "isArguments" check for every array comprehension, which is a bit nasty. It's better to have CoffeeScript generate as little code as possible.

[weepy]

but isn't the problem only ever on the keyword "arguments". Can't we look for that keyword and substitute?

[jashkenas]

I wish.

 func: =>
   something: arguments
   arg for arg in something

Boom.

[jashkenas]

Oh, I'm sorry, what you're suggesting is totally great. I'm just sleep deprived. I'll make a patch for it.

Edit:
It's now on master. Thanks weepy. Closing the ticket.