possible to add object comprehensions ? about coffeescript HOT 7 CLOSED

Yes. I've been thinking about object comprehensions for a while -- the problem is with passing a variable as the comprehension source. Eg:

values: val for val, key in object.

We don't know at compile time if "object" is a JS array or object -- we can generate code that sniffs to tell, but it would probably be long and ugly. Alternately, we could use a different keyword for object comprehensions than "for", but that would be ugly in a different way.

Any ideas?

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do we need to worry about the differences, since objects and arrays basically work the same when enumerated?

1. for k,v in obj

2. for k,v in array
   k would work like each_with_index (nice if the key was an integer rather than a string?)

3. for v in array
   as it is currently

4. far v in object
   would enumerate the values (probably not used much)

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for item in array isn't compatible with Prototype.js, which is a problem. (Otherwise Prototype and CoffeeScript would go real nicely together). Also, the ordering -- for arrays you want the element to be the first argument, followed by the index. For objects you generally want the key to be first, although some frameworks follow array style with the value first and the key optional -- Underscore.js's iterators do this. Not sure how much of a problem any of that is. It would be nice not to completely break Prototype, though.

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I dont see a problem with having the key second - in many ways it makes sense. It would certainly be a shame to break prototype - (grr for adding to Array.prototype!)

I think it might break your 'no std-lib' rule, but we could write some kind of iterator function, that used "instanceof Array" to switch. It might tidy up some of the code (__a etc everywhere), but there might be some ramifications I haven't thought of.

Something along the lines of:

function __iterate(_this, obj, fn, guard) {
  var i;
  
  function run(i, v) {
    with(_this) {
      if(!guard || guard(i, v)
       fn(i,v)
     }
  }

  if(obj instanceof Array) {
    for(i=0; i < obj.length; i++)
      run(i, obj[i])
  } else {
      for(i in obj)
        run(i, obj[i])
  }
}

count(i, v) for i in list

=> 

__iterate(this, list, count)

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or perhaps could use a for .. in loop with a check against obj instanceof Array && !isNaN(index) ??

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or could use obj.hasOwnProperty(index) as a guard. See Resig's post here http://stackoverflow.com/questions/135448/how-do-i-check-to-see-if-an-object-has-an-attribute-in-javascript

This would also guard against methods being added to Object.prototype

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Good idea on the hasOwnProperty -- it'll significantly slow down large comprehensions, but that's ok. Your suggested (for .. in) method is now the way that array/object comprehensions work on master. You can give it a try.

nums: {one: 1, two: 2, three: 3}

counting: key + '!' for val, key in nums

print(counting.join(' '))

Prints: one! two! three!

Closing the ticket... Let me know if you run into any issues and I'll reopen it.

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