`do for item in list` ?

[jashkenas]

An interesting proposal raised by @lehni, here:

#2382 (comment)

... is to somehow combine the do and for keywords to give you a quick and easy way to write a loop that closes over its variables ... without hiding the fact that you're working with an implied inner function, and all of the scope changes that involves. I think it might work out nicely.

do for num in [1, 2, 3]

... becomes the equivalent of:

for num in [1, 2, 3]
  do (num) ->

Any ideas?

[vendethiel]

+1 a lot

[epidemian]

What about

for num in [1, 2, 3] ->

?

At least for me the symbol -> is kind of a synonym for "new scope starts here!".

Edit: Not exactly a synonym. class also introduces a new scope without the need for ->.

[connec]

I would prefer to somehow allow do right after a for expression and/or set the right arguments for the closure as per @lehni:

for num in [1, 2, 3] do ->
  ...

equivalent to

for num in [1, 2, 3] then do (num) ->
  ...

Though I think the existing then do... form provides possibly the best mix of clarity and convenience without adding new syntax or semantics, though it certainly gets a bit duplicative if you're using destructuring etc.

for { id, title, content } in sections then do (id, title, content) ->
  ...

[davidchambers]

I assume that this would work as expected?

do for num, idx in [1, 2, 3]
  console.log 'idx:', idx, 'num:', num

I agree with @epidemian that -> does a good job of announcing new scope. For this reason I find for v in a do -> clearer than do for v in a, but perhaps the latter would feel more natural as it became more familiar. Actually, in the time I've spent writing this comment I've changed my mind. -> as shorthand for a function that receives one argument (or two, if the index is passed in as well) feels too magical. I may be wrong, but I believe that -> not preceded by (...) currently always means this function takes no arguments. I'm warming to do for, in other words.

[ichernev]

+1

Another variant may be

for scoped i in [1,2,3]
  console.log i

similar to for own.

[zeekay]

I am a fan of the syntax proposed by @connec. for num in [1, 2, 3] do -> reads very well.

[joliss]

@jashkenas, I think that's a great idea!

@lehni's and @connec's syntax for num in [1, 2, 3] do -> was what I had tried initially, naively hoping that it would "just work". So to me it seems like the most intuitive variant. I haven't thought about whether it would fit well into the language spec though.

That said, IMHO either syntax would be a great improvement.

[STRd6]

What about:

[1, 2, 3].forEach (num) ->
  console.log i

JavaScript has had iterators for a while now.

[vendethiel]

coffeescript targets IE6.

[vendethiel]

bump after that got asked on irc

for num in [1, 2, 3] do ->
  ...

note that this is valid for a in b do (-> c) then ... is valid

[monolithed]

+1 for:
for num in [1, 2, 3] do -> ...

[nfour]

Any followups? Just found myself writing a loop like this and trying to minimize the amount of indentation thus intuited for key of obj then do ->.

Would prefer if

    for tabKey, apiKey of pageApiMap then do (tabKey, apiKey) ->

Could be shorthanded to

    for tabKey, apiKey of pageApiMap then do ->

Where the do function's parameters are implicitly passed when you don't specify them; because I can't see a scenario where you wouldn't want the parameters passed to a new anonymous function.

[connec]

You could almost 'merge' for and function syntax into something like

for (key, value) of object ->
# for key, value of object then do (key, value) ->

for (value, index) in array ->
# for value, index in array then do (value, index) ->

for (key) of object ->
# for key of object then do (key) ->

for (value) in array ->
# for value in array then do (value) ->

# This is perhaps a bit too interesting
for (value, index, a = 1) in array ->
# for value, index in array then do (value, index, a = 1) ->