Proposal: Elision

[danielbayley]

Following on from #3972 (and this comment in particular)…

Should we not align with ES6 and implement elision?

const [,,c] = [1,2,3];
console.log(c) // 3

[,,c] = [1,2,3]
console.log c # Error

[jashkenas]

Yes, as soon as we compile into ES6 destructuring, we should allow it.

(Distasteful as it may be)

[GeoffreyBooth]

We already are for arrays.

[vendethiel]

@GeoffreyBooth It's about elision, so [,,thing]

[GeoffreyBooth]

Yes. We don’t currently support elision (for anything) but as of 2.0.0-beta5 we output into ES syntax for array destructuring; and into ES2015 (fallback) syntax for object destructuring, until object destructuring reaches Stage 4 in ES2017 or ES2018.

So to support elision, we could support it for array destructuring now and output like the example above; but for object destructuring, if that even makes sense for elision, we would need to output into Stage 4-compatible syntax like how object destructuring is output now.

[danielbayley]

(Distasteful as it may be)

😄 I think this is one of those problems to which there is no real stand-out/elegant solution. If you just give the unused params a name that will throw linter errors when [as you ideally would be] checking for unused variables in general.

It does feel more elegant to me than other suggestions Iv'e seen… and of much more use than say, trailing commas in arrays.

Also, I wonder if this should be a thing with function parameters? (for consistency)

fn = (,,c) => console.log c
fn 1,2,3 # 3

[connec]

Don't forget that 'expansions' are a thing in CS:

[..., c] = [1, 2, 3]
((..., c) -> c) 1, 2, 3

[jashkenas]

Trailing commas in arrays are a good example of why elisions are kinda stinky, syntax-wise.

Because if: [,b,] = [1,2,3] destructures a three-element array ... then [1,2,].length ought to equal 3. But it doesn't.

[GeoffreyBooth]

Because if: [,b,] = [1,2,3] destructures a three-element array … then [1,2,].length ought to equal 3. But it doesn’t.

But destructuring and literal syntaxes aren’t equivalent. { x } = obj is not obj = { x }. Your [1,2,].length is a literal syntax.

Besides, is this something we need to worry about? We can just compile more or less one-to-one with ES, and let the runtime handle it however the ES committee has decided these cases should go.

[jashkenas]

Besides, is this something we need to worry about?

It's not. We should support elision. I'm just grumbling a little.

[danielbayley]

if: [,b,] = [1,2,3] destructures a three-element array

The trailing comma here is entirely superfluous though, so not sure why anyone would actually write that? Elision is only useful for skipping arguments… to discard the rest one just ends the destructuring early.

[1,2,].length ought to equal 3

Not sure I agree here… to me 2 is correct as this reads as having only 2 actual items. Although having said that; [1,2,undefined].length is 3…

Maybe trailing commas are the 'bad part' here as opposed to elision.

[danielbayley]

Don't forget that 'expansions' are a thing in CS:

[..., c] = [1, 2, 3]
((..., c) -> c) 1, 2, 3

@connec A fine thing! But not so handy when you want to do:

[,second,,fourth] = [1,2,3,4] # Don't need 1 or 3

[GeoffreyBooth]

@connec A fine thing! But not so handy when you want to do:

[,second,,fourth] = [1,2,3,4] # Don’t need 1 or 3

Yes, but I think @connec’s point is that how common is the case where you don’t want the first or last? Because the old-fashioned:

elements = [1,2,3,4]
second = elements[1]
fourth = elements[3]

Can always work for those edge cases. I get that the whole point of elision is that it’s a convenience shorthand, that it doesn’t add anything we can’t do already, but I guess that question is how much more useful is it than simple expansion? How common are the cases that elision would help with that expansion doesn’t already cover?

[danielbayley]

@GeoffreyBooth Sure. Obviously my example was contrived… the main one I keep running into in reality, where I think elision would be nice, is with: [,stuff,from] = string.match // which is fairly common (for me anyway).

[GeoffreyBooth]

Sure. That's the only use case I can think of either, but it's certainly common. I would be fine with adding this unless it presents other issues.