for comprehension doesn't works with return and yield

[al6x]

This code doesn't compiles

f = ->
  return for v in list
    v

[mklement0]

It will compile (at least on 1.7.1) if you align the last line (v) with the one above, but the return for v in list doesn't make sense, as it will return (and thus exit the function) after the first iteration (you're effectively saying: "return for each element of list", which, of course, only returns the first element).

You probably want:

f = ->
  v for v in list

Or:

This relies on CoffeeScript's implicit returns combined with the [almost]-everything-is-an-expression philosophy - the explicit, non-idiomatic equivalent would be return (v for v in list).

(Of course, unless you plan on actually operating on the elements in list, you may as well just do f = -> list.)

Edit: You probably meant the prefix form of the comprehension; the approach is the same, though:

f = ->
  for v in list
    v

[al6x]

You probably want: v for v in list

Exactly, I want to return another list after doing some modifications to v in loop. But since it's possible to write it this way return v^2 for v in list would be nice to have it also for

return for v in list
  v^2 

It's not a "new line" after return. There's an indentation, it's part of the loop, not a "new line".

You probably meant the postpositional form of the comprehension; the approach is the same, though:

Yes, and as soon as it works with implicit return - it looks strange why it stop working when you put that return explicitly.

[mklement0]

Pragmatically speaking:

• In the spirit of CoffeeScript: I suggest not using return - see my last code snippet above.
• If you still want to, enclose the entire for expression in parentheses.

In case you're raising a language-design question: I'll let others who know more about the language answer, but I can say this: at the very least there is ambiguity: return for v in list is by itself a complete statement, and that's what the compiler currently assumes.

(As an aside: maybe you're fully aware of this, but note that ^ is the XOR bitwise operator in CoffeeScript/JavaScript; perhaps you meant v*v or, more generally, Math.pow v, 2.)

[michaelficarra]

@mklement0: When followed by a block, the meaning changes. Also consider return for v in list then v ** 2. Also note the ** exponentiation operator, added in 1.7.0.

[mklement0]

@michaelficarra: Thanks for the tips - very handy.

return for v in list then v ** 2 actually breaks (in 1.7.1) without parentheses.

When followed by a block, the meaning changes.

Can you please elaborate? Are you saying that what @alexeyPetrushin is trying to do should work?

[michaelficarra]

Yes, which is why I reopened this issue. No parentheses should be needed when a block follows the comprehension header. These examples all work in CoffeeScriptRedux.

[mklement0]

@michaelficarra: Got it, thanks. So return for v in list then v ** 2 breaking without parentheses is a manifestation of the same bug?

[michaelficarra]

Yep.

[xixixao]

Related: throw for v in list then v compiles to invalid JS.

[al6x]

@xixixao seems like inconsistent behaviour of throw is by design #3347

[xixixao]

That's a different issue. We shouldn't be generating invalid JS.

[GeoffreyBooth]

The initial example returns an “unexpected indentation” error in CS2, so I think that resolves this particular issue.

Added @xixixao’s invalid JS example to #3709, which appears to be related if not identical.