propose a functional let grammar

[winterland1989]

Here is my little proposal to bring let into coffee:

• let ... in statements

zipWith = (xs, ys, fn) ->
  result = []
  let xl = xs.length, yl = y.length in
    if xl >= yl
       ...
    else 
       ...
  result

will be compiled to

var zipWith = function(xs, ys, fn){
  var result = []
  {
    let xl = xs.length, yl = y.length;
    if(xl >= yl){
       ...
    }else{
       ...
    }
  }
  return result;
}

• let ... in expressions

always turn them into a IIFE, just like a do notion.

• special for let

for let i in [1..100]
  ...

will be compiled to

for(let i = 1; i<=100; i++){
  ...
}

[michaelficarra]

A few notes:

• CoffeeScript will not be targeting ES2015 any time soon, so we will emit IIFEs for the foreseeable future

• We used to automatically capture loop variables that were closed over. Now, users are expected to manually capture them using this syntax, which isn't far off from what you propose:

  for a in b then do a =>
    ...

A related tip: when making proposals like this, try to keep them limited to the basic functionality that you require. Adding on these semi-related concepts only confuses the message and distracts conversation, ultimately hurting your proposal's chances. The only thing I'd recommend including with your proposal next time is usage examples.

[vendethiel]

There's an issue about for(let: #2518

[michaelficarra]

Good, then we can safely ignore that part of this proposal. On the remaining let a = b in ... part, I prefer it to do (a = b) => ... syntactically, but I don't know if that is enough to warrant adding what is essentially sugar for something we already have.

[winterland1989]

Sorry for not bringing such simple idea clearly, my original purpose is trying to bring es2015 feature let into language, after reading your suggestion, it looked like it's better to directly add let..in and for let and translate to let version, just like how we bring into yeild, use it or not is depended on user's own choice.

You can continue use for a in b then do a => to emit an IIFE if you want, this proposal is just a clean way to write let in coffee IMO.

BTW, i don't miss any other features in es2015 anyway, but let is a nice feature IMO, so let's do this : )

[winterland1989]

Proposal updated, does it look ok to you?

[GeoffreyBooth]

http://coffeescript.org/v2/#unsupported-let-const