backslash line continuation pre-object breaks objects

[mjbaldwin]

I tend to use backslash line continuation liberally, since I've been bitten by a lot of times when CoffeeScript breaks without it, for example this breaks if any of the backslashes are missing:

switch condition
  when \
      "long first condition on one line when I want to limit line length to 80 chars", \
      "second condition on a third line", \
      "third condition, different lines for readability"
    doSomething()

But now I'm being bit by the opposite, I tried writing:

                callFunction arg1, \
                  object_key_1: data1 
                  object_key_2: data2

But it compiles, bizarrely, to:

callFunction(arg1, {
  object_key_1: data1
});

({
  object_key_2: data2
});

I can't imagine why it's splitting up the object. Removing the backslash produces the desired behavior:

                callFunction arg1, 
                  object_key_1: data1 
                  object_key_2: data2

produces

callFunction(arg1, {
  object_key_1: data1,
  object_key_2: data2
});

Now, backslashes aren't even a documented functionality of CoffeeScript, at least not that I can find on the website, despite the fact that they're sometimes required for multi-line functionality (as in the switch above). And I can't figure out for the life of me when they're required, and when they're not, except through trial-and-error.

But the fact that they break existing code when they're added in places where they should just be redundant, seems like a bug, unless there's something additional that backslashes mean that I'm missing.

Basically, shouldn't I be able to add a backslash to the end of any line (except perhaps object property lists, and of course multiline strings), without it changing the meaning of my code? So I'm filing this as a bug... Please correct any misunderstandings I may have here, though!

[michaelficarra]

I think you have a big misunderstanding about backslashes. Backslashes cause the following newline to be ignored. Would you expect

callFunction arg1,                   object_key_1: data1 
                  object_key_2: data2

to compile? Of course not.

[mjbaldwin]

Ah, got it.

Was a misunderstanding indeed -- I thought backslashes just "forced" the next line to be interpreted as part of the same statement, not that they literally removed the newline from parsing.

Is there a way to get this backslash behavior officially documented? I'd do a whole pull-request thing with the website and add it myself, but I don't know if that's the preferred way or not for CS.

Anyways, thanks, closing the issue.

[michaelficarra]

Pull requests are always welcome. Whether or not we want line-continuing backslash in the documentation is a different question.

[mjbaldwin]

Right, I'd be happy to add it, but only if it's wanted.

My $0.02 is that I wish the CS docs were much more comprehensive; I feel like half of what I use day-to-day in CS is undocumented and was learned from frustrating trial and error, and a lot of time could have been saved if things like backslashes were in the docs, since they're clearly a core and necessary language feature to write things that can't be achieved any other way (like the switch statement at the top of this issue).

[michaelficarra]

See #1808 and #1631 to start.

[jashkenas]

... for what it's worth, the reason they're not in the docs is because you're not really supposed to ever have to use them. I'd write your initial example like this:

switch
  when condition(a)
    doSomething()

... where condition is an extracted function that does what you want.

[mjbaldwin]

Hi Jeremy, thanks for the suggestion.

First of all, I didn't even know you could use a switch statement without a parameter after the switch -- another undocumented feature! :)

Although I feel like hiding the switch values inside a function defeats the whole purpose of the switch statement, which is to be able to compare a single values to other sets of values in a straightforward (and, above all, visible) way.

To write a maintainable solution without using backslashes, I suppose I'd have to rewrite the whole switch as a big if statement. Oh well, thanks for the feedback!