let for

[vendethiel]

jashkenas/coffeescript#2518

let for looks right to me, or for let

[satyr]

for let maybe.

[vendethiel]

Yes, for let sounds more correct syntax-wise - let for sounds more "english like" - but I know you believe english, or any natural language, is total crap. But I feel like it "adds" to the for syntax, without need.

[vendethiel]

Any +1/-1 ? I think it's a regular use case (please let for).

[satyr]

FWIW, ES6 version will look like: for (let x of xs)

[vendethiel]

I doesn't matter much, both syntaxes are ok. True with the parenthesis it looks as nice, without a bit less - but I'd still be as happy.

[satyr]

More analysis

We already got idioms using let and for together:

1. let+for: Localizing the loop variables

   let
     for x, i of xs
       ...
   

2. for+let: Creating fresh bindings for each iteration

    for x, i of xs
     let
       ...
   

• The proposal may need to do both 1 and 2 efficiently to be worth a new construction.
• let for as proposed is clearly confusing due to 1.
  • for let it is.
    • for let own or for own let, or both?

[vendethiel]

This does not work :

for [x,y,z] of b
  let
    x + y + z

For let would capture any loop variable, even the destructurations
let own/own let it depends if the own is seen as a for construct or applicated to the variable, I suppose.

[satyr]

This does not work

Nope, because auto-binding all of them is costly and may not be desirable (and you can always do for b => let [x, y, z] = & => ...).

depends if the own is seen as a for construct or applicated to the variable, I suppose

Both, I suppose.

[vendethiel]

That's what the suggestion is for. Capture them. Maybe you can consider searching them in the block, like that/if. May be costly too, but not at runtime.

[satyr]

Capture them

Naturally it will, since it won't have to expose the bindings to the outer scope as well.

for let bindings, i of xs => ...

will compile roughly as:

for tmp, tmp-i of xs
  let bindings = tmp, i = tmp-i
    ...

[vendethiel]

Which is what we're looking for, isn't it ?

[vendethiel]

Okay, I have something.
It's super ugly, super unoptimized etc etc. I wanted to use Call.let but I didn't managed to make it work :(.
d3223ce, cleaned up (a tiny bit) (allowing let for a, b of c then) by 44a0c90
Should work with makeReturn.

It got over my head really fast, I guess I need far more experience haha

[vendethiel]

My diff is here. (still a @idx = left)
That's still pretty bad as I have basically no idea how coco works, but at least I don't feel so much shame when reading it.
It currently messes up the indentation, tho.

$ node lib/command -bce "let for a in b then"
var a;
for (a in b) {
(fn$.call(this, a))
}
function fn$(a){}

• Not indented
• Misses the ;
  is it because of LEVEL_TOP?