CS2 Discussion: Output: Fix the scoping of the for loop

[mitar]

Should we do in the new version CoffeeScript something about the scoping of the for loop and variables being reused inside the closures? I have been bitten so many times with this. CoffeeScript in general tries to fix such strange things from JavaScript, maybe it is time that this is also fixed?

[edemaine]

#62 partially addresses this (but see the limitations). I personally can't imagine a more backwards-compatible fix, but I think we're all open to specific suggestions!

[mitar]

I mean, we are breaking compatibility with new CoffeeScript, no?

I would simply do: if a variable from for loop is used inside any closure inside of it, it is automatically wrapped inside a inline function call (do).

I cannot imagine a use case where you would want to use variable from the for loop inside the closure with the traditional scoping.

[edemaine]

@mitar Here's one example: (hopefully not too contrived)

for x in array
  show = ->
    console.log 'Current value of x is', x
  show()
  ... do some stuff, change x ...
  show()
console.log 'After the loop, x is', x

As I understand your proposal, the second show would fail, printing the old value of x instead of the new one. The behavior would be even worse/less intuitive if the show function changed the value of x (but maybe you want to avoid do in that case...).

[JavascriptIsMagic]

Especially if #61 is to happen, I would like to propose changing the behavior of do for to output for (let for scoping:

array = [1, 2]
value = 'outside'
do for value in array
  console.log 'Inside:', value
console.log 'Outside:', value

Which would compile to something like:

var array = [1, 2]
var value = 'outside'
for (let value, index = 0; index < array.length; index++) {
  value = array[index]
  console.log('Inside:', value)
}
console.log('Outside:', value)

Currently in coffeescript if you try to use any form of do for it will actually compile to javascript, but will always throw an error at runtime:

Uncaught TypeError: (intermediate value)(intermediate value)(intermediate value)(intermediate value)(intermediate value)(...) is not a function

Because it will try to call ([])() an array as if it where an anonymous function.
You can try it out here.

It is important to note that this output change should not effect using do inside a one line for loop:

function1 = -> console.log 'function 1'
function2 = -> console.log 'function 2'

do fun for fun in [function1, function2]

Try it here

We should only change the output for do for if do is immediately followed by for

---

Very recently I actually ran into this exact problem, and although my code was much more complex it basically boiled down to:

value = 'outside'
[inside1, inside2] =
  do (value) ->
    for value in [1, 2]
      do (value) ->
        -> console.log 'Inside:', value
console.log 'Outside:', value
inside1()
inside2()

# Output:
Outside: 'outside'
Inside: 1
Inside: 2

Try it out

[mitar]

@edemaine: No, this should work.

I would compile your:

for x in array
  show = ->
    console.log 'Current value of x is', x
  show()
  ... do some stuff, change x ...
  show()
console.log 'After the loop, x is', x

To:

for x in array
  do (x) ->
    show = ->
      console.log 'Current value of x is', x
    show()
    ... do some stuff, change x ...
    show()
console.log 'After the loop, x is', x

As you see, you can change x as it is.

@JavascriptIsMagic: Interesting, I didn't know that let inside for loop in JavaScript now rebinds for every loop. Maybe we should just make this a default? So for loop should always use let?

[edemaine]

@mitar Ah, right. But there's still a problem: after the loop, you'll print the wrong value of x.

[GeoffreyBooth]

Closing as this wasn’t changed for 2. But we should perhaps open a new issue, maybe on the main repo, for potentially providing some non-breaking-change way of declaring a loop iteration variable with let. See #58 (comment)

[coffeescriptbot]

Migrated to jashkenas/coffeescript#4956