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  • function.next meta-property proposal

function.next meta-property proposal

# Allen Wirfs-Brock (10 years ago)

At the March TC39 meeting we agreed that the function.next metapropty should be made into a standalone stage 1 proposal.

That proposal is now available at: allenwb/ESideas/blob/master/Generator metaproperty.md

# Alexander Jones (10 years ago)

In Python, sending a value other than None into the first invocation of send raises an error. That seems like a reasonable behaviour to me, without introducing too much weirdness.

TypeError: can't send non-None value to a just-started generator
# Kevin Smith (10 years ago)

Alexander, ES6 generators accept any arbitrary values for the first invocation of "next". That's not going to change.

# Alexander Jones (10 years ago)

I don't think that's a good enough reason by itself to make this observable. You can pass arbitrary numbers of arguments to any function in JS, and they are generally ignored.

# Andrea Giammarchi (10 years ago)

Alexander ES6 "is out" as it is, here the proposal is to make reachable that argument because indeed, since about ever, in JS you can ignore arguments, but you can also always reach them through the argument object.

When it comes to generators, arguments object would be a misleading place to give you any sent value 'cause that's rather how you initialize the generator, hence the function.nect proposal, and I must say I like it in all its weird glory.

Like you said, Python throws, meaning devs coming from Python won't ever make such mistake of sending values a tthe very first .next invoke, so JS early adopters know thanks to all bounce of early articles regarding the early specs :D

My only concern, is that the "famous" Promise based generator wrap www.promisejs.org/generators/#both needs an update:

function async(makeGenerator){
  return function (value) { // <= here
    var generator = makeGenerator.apply(this, arguments);

    function handle(result){
      // result => { done: [Boolean], value: [Object] }
      if (result.done) return Promise.resolve(result.value);

      return Promise.resolve(result.value).then(function (res){
        return handle(generator.next(res));
      }, function (err){
        return handle(generator.throw(err));
      });
    }

    try {
      return handle(generator.next(value)); // <= here
    } catch (ex) {
      return Promise.reject(ex);
    }
  }
}

Best

# Andrea Giammarchi (10 years ago)

actually, that won't work ... so yeah, that pattern is somehow compromise, arguments is used to indeed initialize the generator, no way to put first value in ... oh well