Independent splats in array pattern matching

[TrevorBurnham]

Let's say that I want to get the first and last values of an array of arbitrary length, in a nice one-line statement. I could write

[first, middle..., last] = arr

But this is less than ideal from both a code efficiency standpoint and a readability standpoint, since I never use middle. What I'd prefer to write is simply

[first, ..., last] = arr

Do others agree that this should be allowed?

[satyr]

Not as nice, but you can write:

{0: first, (arr.length - 1): last} = arr

[michaelficarra]

I don't see why not. Technically, it doesn't actually make sense to introduce a middle variable if it is never intended to be used. It hurts the readability of the code. I'd be in favor of this as long as nobody can spot any glaring issues.

[satyr]

You'd want a placeholder syntax that takes one space as well.

$ coffee -bpe '[first, _, third, _, fifth] = a'
var fifth, first, third, _;
first = a[0], _ = a[1], third = a[2], _ = a[3], fifth = a[4];

In SpiderMonkey, empty entries serve well.

js> [first, , third] = [1, 2, 3]
1,2,3
js> [first, third]
1,3

[michaelficarra]

@satyr: good idea. I'd be in favor of the empty entries. The first syntax you mentioned would work fine right now if you didn't care about the value of the _ variable, but definitely shouldn't be introduced as a syntactic construct.

[satyr]

Of course though,

{0: first, 2: third, 4: fifth} = a

works just as well.

[StanAngeloff]

Ah this comes back to life. I need to dig up the old discussion we had on the topic.

EDIT: Start with #86, move on to #277.

EDIT 2: Thought I'd share http://githubissues.heroku.com/#jashkenas/coffee-script/ is great for searching.

[TrevorBurnham]

Yes, I like weepy's proposal at issue 277 of using ? as a single-value placeholder. It looks very clear to me:

[first, ?, third, ?, fifth, others...] = arr

[TrevorBurnham]

In fact, it'd be cool to see ? (or some other character) as a no-I-don't-need-this-value placeholder in other contexts in CoffeeScript as well; for instance, sometimes I just want the values of a hash, not the keys:

foo(val) for ?, val of hash

I understand the "named values contribute to more self-documenting code" argument Jeremy gave in issue 277, but I feel like that benefit is usually outweighed by the clarity that comes with avoiding the declaration of a variable you never use. For instance,

[first, ..., last] = arr

makes it much more clear that you only care about the first and last values of the array than any possible name for the middle value would. And, of course, there are the side benefits of brevity and slightly more efficient JavaScript output.

Would someone like to submit a patch?

[satyr]

What would arr = [first, ..., last] produce?

[TrevorBurnham]

A syntax error. It's not necessary for the pattern-matching syntax to be completely parallel to the array creation syntax.

[jashkenas]

I don't think we need this for 1.0 ... Even if you're not going to use a variable that's serving as a placeholder, at least you can name it something descriptive. The only place that placeholder syntax would be used is in pattern matches... So, closing as a wontfix.

[satyr]

[first, ..., last] = arr

[first, ?, third, ?, fifth, others...] = arr

Coco now supports both of those without needing this proposal, in the form of

{0: first, (*-1): last} = arr
[first, [], third, [], fifth, ...others] = arr

[jashkenas]

satyr: want to link to your patch?

[TrevorBurnham]

Could we reopen this issue? It was closed with the comment "I don't think we need this for 1.0"; now that 1.0 has successfully been released, I believe it merits further discussion.

Specifically, I'd like to propose allowing

[first, ..., last] = arr

and

[first, ?, third, ?, fifth] = arr

Here's a common use case for the latter: I run a regex, and I only want the group matches (or perhaps a subset of group matches), not the full match. For instance, let's say that I have a coordinates string in the format x,y, where x and y are integers. So I'd like to be able to write

[?, x, y] = coordinates.match /(\d+),(\d+)/

The closest I can come with the current syntax is to either 1) put in an unnecessary variable name instead of ?, or 2) write

[x, y] = coordinates.match(/(\d+),(\d+)/)[1..]

The ? syntax is, I think, both more readable and more writable, and would generate more efficient code.

Similarly, if I have a function call foo(bar) that returns a list of values, of which I only want the last three, I think

[..., x, y, z] = foo bar

is clearer than any existing syntax, and would generate more efficient code than any existing one-liner.

[michaelficarra]

I am in support of the ... syntax, not so much the ?. Though I don't really have a better suggestion, so I'd be okay with it. With regards to your last example, though, I think [x, y, z] = (foo bar)[-3..] is pretty clean. Though I guess the new proposed syntax is more readable and more naturally understandable to people unfamiliar with coffee.