Arguments placeholders for callbacks

[mixty1]

It would be nice, if you add to civet something like this

I have this code:

export default function throttle(func: (...args: any) => () => void, ms: number)

  isThrottled .= false
  savedArgs: any | null .= null
  savedThis: any | null .= null

  function wrapper(this: any): void
    if isThrottled
      savedArgs = arguments
      savedThis = this
      return

    func.apply this, arguments

    isThrottled = true

    setTimeout =>
      isThrottled = false
      if savedArgs
        wrapper.apply savedThis, savedArgs
        savedArgs = savedThis = null
    ,ms

  wrapper

And I would like to have such an opportunity as:

export default function throttle(func: (...args: any) => () => void, ms: number)

  isThrottled .= false
  savedArgs: any | null .= null
  savedThis: any | null .= null

  function wrapper(this: any): void
    if isThrottled
      savedArgs = arguments
      savedThis = this
      return

    func.apply this, arguments

    isThrottled = true

    setTimeout(&, ms) =>
      isThrottled = false
      if savedArgs
        wrapper.apply savedThis, savedArgs
        savedArgs = savedThis = null

  wrapper

someFunc(&, arg1, arg2, &, arg3)
  (argsFromFirstPlaceholder) =>
    // ...some code here
  (argsFromSecondPlaceholder) =>
    // ...some code

[edemaine]

I hadn't realized it before, but the one-argument case is a special case of #480, which increases the motivation for that proposal. It looks like imba also only supports one argument?

The two-argument version is interesting. Indentation application already does this:

someFunc
  (argsFromFirstPlaceholder) =>
    // ...some code here
  (argsFromSecondPlaceholder) =>
    // ...some code
↓↓↓
someFunc(
  (argsFromFirstPlaceholder) => {
    // ...some code here
  },
  (argsFromSecondPlaceholder) => {
    // ...some code
  }
)

So what this suggests is that someFunc(&, arg1, arg2, &, arg3) should map to ($1, $2) => someFunc($1, arg1, arg2, $2, arg3). Seems a reasonable generalization, though it's maybe a little counterintuitive that the &s don't refer to the same parameter; see #85 for possible alternatives.

[mixty1]

It looks like imba also only supports one argument?

Probably yes.
By the way, if i'm not mistaken swift lang has similar syntax, but without placeholders, just callback after function call or something like that.

So what this suggests is that someFunc(&, arg1, arg2, &, arg3) should map to ($1, $2) => someFunc($1, arg1, arg2, $2, arg3)

With defined default parameter with callback functions:

($1 = (argsFromFirstPlaceholder) => { /* ...some code here */ }, $2 = (argsFromSecondPlaceholder) => { /* ...some code */ }) => someFunc($1, arg1, arg2, $2, arg3)

from this code:

someFunc(&, arg1, arg2, &, arg3)
  (argsFromFirstPlaceholder) =>
    // ...some code here
  (argsFromSecondPlaceholder) =>
    // ...some code

Seems a reasonable generalization, though it's maybe a little counterintuitive that the &s don't refer to the same parameter;

Maybe for that case, if has multiple different placeholders, then numerate it like:

someFunc(&1, arg1, arg2, &2, arg3)
  (argsFromFirstPlaceholder) =>
    // ...some code here
  (argsFromSecondPlaceholder) =>
    // ...some code

or add labels:

someFunc(&cb1, arg1, arg2, &cb2, arg3)
  (argsFromFirstPlaceholder) =>
    // ...some code here
  (argsFromSecondPlaceholder) =>
    // ...some code

[edemaine]

Looking back at this, the idea seems close to what's offered by . placeholders (#1070). In particular, we have:

(setTimeout ., 1000)
  => console.log 'hello world'
---
(($) => setTimeout($, 1000))(() =>
  console.log("hello world"),
);

Unfortunately, currently:

• The parentheses are required.
  • Without any parens, the arrow function is an argument to 1000. This unfortunately seems "right" according to the low precedence of implicit function application.
  • With parens around just the arguments, the . lifts outside the second call. I think this is a bug (. placeholder lifts outside long string of calls #1449).
• . doesn't work with multiple arguments. For example, setTimeout ., . means $ => setTimeout($, $), not ($1, $2) => setTimeout($1, $2). Potentially this is just the WIP state, though, and we could generalize (though it's not mentioned in & function shorthand revamp, and ./... argument placeholders #1070 — it only mentions &1, &2, but .1 and .2 are numbers).