Closures do not close in comprehensions

[lsb]

In the REPL on coffeescript.org:

comprehended = ((() -> alert i) for i in [0..10])
mapped = [0..10].map( (i) -> ( () -> (alert(i)) ));

comprehended[5]()  // 11
mapped[5]()  // 5

This is unintuitive.

[paulmillr]

You need to use do notation to preserve context. In this case:

comprehended = ((do (i) -> -> alert i) for i in [0..10])

[michaelficarra]

Yeah, this is consistent with JS behaviour. CS used to close over loop iterators when defining functions within the comprehension, but it lead to sometimes very complex compilations, and would be equally unexpected behaviour for someone expecting our current, more straightforward compilation. Recommended style is now:

comprehended = ((do (k = i) -> -> alert k) for i in [0..10])

or

comprehended = ((do (i) -> -> alert i) for i in [0..10])

or

comprehended = (((i) -> -> alert i)(i) for i in [0..10])

[lsb]

Oh, no question, lexical scope makes for less straightforward compilation, compared to dynamic scope; I just assumed that the scoping rules would be consistently lexical. Do you happen to remember where the commit to change that was? That might be interesting to see.

[michaelficarra]

See #1271, #959, #423.