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  • Proposal: Placeholder operator

Proposal: Placeholder operator

# Sultan (5 years ago)

Placeholder operator: !

Use in function parameters(maintains arity without creating a named binding):

const foo = (a, b, !) => {}

Use in de-structuring(allows you to capture further along a tuple without creating a named binding):

const [!, setState] = useState(0)

# dante federici (5 years ago)

Any reason you don't like the empty space with a comma? I also don't, but that's what it is now:

arr.map(( , index) => ...)

Also wouldn't mind ? or , since I associate ! with "not" as a unary operator, but "" isn't a unary in JS.

# Claude Pache (5 years ago)

Le 11 janv. 2019 à 14:02, Sultan <thysultan at gmail.com> a écrit :

Placeholder operator: !

Use in function parameters(maintains arity without creating a named binding):

const foo = (a, b, !) => {}

Today, you can write:

const foo = (a, b, _) => { }

Is the complexity added to the language worth the feature?

Use in de-structuring(allows you to capture further along a tuple without creating a named binding):

const [!, setState] = useState(0)

You can already write:

const [ , setState] = useState(0)
# Sultan (5 years ago)

empty space with a comma?

I think that only works with trailing params. For example this is not possible:

const foo = (a, , c) => {}

Today, you can write: const foo = (a, b, _) => {}

However that does throw with:

const foo = (a, _, _) => {}

You can already write: const [ , setState] = useState(0)

Thanks i forgot about that.

# Tab Atkins Jr. (5 years ago)

On Fri, Jan 11, 2019 at 6:42 AM Sultan <thysultan at gmail.com> wrote:

empty space with a comma?

I think that only works with trailing params. For example this is not possible:

const foo = (a, , c) => {}

It doesn't even "work" with trailing params. Function arglists can contain a trailing comma, it's just ignored and does nothing. const foo = (a, b, )=>a+b still has a .length == 2, and (a, b, ,)=>a+b is

just invalid syntax. The trailing comma is just allowed so you can format a long arglist onto separate lines, and don't have to remember that the last argument doesn't have a comma.

Today, you can write: const foo = (a, b, _) => {}

However that does throw with:

const foo = (a, _, _) => {}

Yup, I've run into that problem before, and had to hack around by using _1 for the second ignored arg. It's rare/niche enough that I'm not sure it's worth solving, but I do acknowledge it as an annoyance.

You can already write: const [ , setState] = useState(0)

Thanks i forgot about that.

Yeah, using holes works, but it's definitely harder to read than using _.

And the fact that the last trailing , does not create a hole isn't relevant for destructuring, but it would be relevant and confusing for function-arity if we allowed holes in arglists. That is, [1,2,].length returns 2 list, while [1,2,,].length returns 3. If we get pattern-matching and array literals can do strict length checking, it would also become relevant and confusing. (But that's several stacked "if"s!)