partial spread syntax

# 月の影 (6 months ago)

Sometimes I got a infinity iterable sequences, I may want a partial spreed syntax. ...iterableObject{from, to}

For example:

function *fibonacci() {
    let a = 0, b = 1
    while(1) {
        [a, b] = [b, a + b]
        yield a
    }
}


console.log([...fibonacci(){3, 5}]) // [3, 5, 8]

For a finite list, it is similar to [...list].slice(from, to)

const arr1 = [1, 2, 3, 4],
    arr2 = [5, 6, 7, 8]


console.log([...arr1{2}, ...arr2{1}])  // 3, 4, 6, 7, 8
# Tab Atkins Jr. (6 months ago)

On Sun, Mar 25, 2018 at 6:20 PM, 月の影 <19511344 at qq.com> wrote:

Sometimes I got a infinity iterable sequences, I may want a partial spreed syntax. ...iterableObject{from, to}

For example:

function *fibonacci() {
    let a = 0, b = 1
    while(1) {
        [a, b] = [b, a + b]
        yield a
    }
}

console.log([...fibonacci(){3, 5}]) // [3, 5, 8]

For a finite list, it is similar to [...list].slice(from, to)

const arr1 = [1, 2, 3, 4],
    arr2 = [5, 6, 7, 8]

console.log([...arr1{2}, ...arr2{1}])  // 3, 4, 6, 7, 8

This seems to be well-served by a simple library function right now:

function take(n, iter) {
  if(n <= 0) return;
  for(let item of iter) {
    yield item;
    if(n-- <= 0) return;
  }
}

function drop(n, iter) {
  iter = iter[Symbol.iterator];
  for(let i = 0; i < n; i++) {
    iter.next();
  }
  yield* iter;
}

function subiter(start, end, iter) {
  return take(end-start+1, drop(start, iter));
}

console.log([...subiter(3, 5, fibonacci())]);
# Mike Samuel (6 months ago)

On Sun, Mar 25, 2018 at 9:20 PM, 月の影 <19511344 at qq.com> wrote:

Sometimes I got a infinity iterable sequences, I may want a partial spreed syntax. ...iterableObject{from, to}

How would you prevent ambiguity between the spread and blocks?

iterable { from, to };

is currently equivalent to

iterable; { from, to; }

# Michael J. Ryan (6 months ago)

I'm not sure I'd prefer this over just using a function on the Array prototype... you aren't saving too many characters and are adding some ambiguity with to {} usage. I don't like it.