Iterators (ES6)

[ahejlsberg]

Creating an issue to track work on iterators. This includes:

• Finalize Iterable and Iterator types.
• Support iterators with spread operator (...) in function calls and array literals.
• Support destructuring of iterator values.
• Support for...of with iterator values.

[JsonFreeman]

Also, when we do generators, we have to support yield* on an iterable.

[DanielRosenwasser]

I've made each of the bullet points into a task list item so that we can check these off and keep appropriate track of them.

[JsonFreeman]

Iterables and Iterator types are addressed by #2083

[JsonFreeman]

ES6 level for...of is handled by PR #2164.

[JsonFreeman]

@ahejlsberg I have a question that may be relevant here, though it is not required. Should spread elements support contextual typing? By this I mean two things:

1. If an array is contextually typed by T, any spread element inside gets contextually typed by T, rather than the element type of T.
2. If a spread argument is contextually typed by T[] (because it aligned with a rest parameter T[]), then its operand gets contextually typed by T[] rather than T.

My gut says that this is not necessary. The only reason I bring it up is that Number 2 above could make type argument inference stronger.

[JsonFreeman]

Actually, thinking about this more, I think it would make things more fluid and actually make things more consistent.

[JsonFreeman]

My plan for spread and destructuring is as follows:
To type check a spread expression ...expr with language target ES6:

• The operand has to be iterable (assignable to the Iterable<T> interface, where T given by the type of expr[Symbol.iterator]().next().value)
• If it is a SpreadElement, the surrounding array gets that T as one of its element types.
• If it is a SpreadArgument, then that T must be assignable to the element type of the rest parameter it aligns with. It will also be possible to do type argument inference from that T to the element type of the rest parameter it aligns with.

For array destructuring (both binding and assignment varieties), we will type each destructuring array element in the following way when target is ES6:

• The destructuring parent type must be assignable to Iterator<T> in the same way as spread.
• For non-rest elements:
  • For the element at index i, we take the property named "i" from the destructuring parent type.
  • If there is no such property, then the element gets associated with the iteration type T from above (just like spread).
• For a rest element, the type is T[] instead of T.

The key difference between the two is that destructuring prefers the numerically named properties of the parent type, whereas the spread argument doesn't care about them.

Note that the type rules for ES3/ES5 don't change, the above are just the type rules for ES6.

[corporatepiyush]

eagerly waiting for this feature to use on io.js. Any tentative release date ?
Would we able to use that version of typescript to compile down to ES3 or ES5 for IE9 and IE10 ?

[JsonFreeman]

@corporatepiyush, the only thing left for 'for...of' loops is #2308. Once that's in, you can use those loops.

Type checking for iterate on spread / iterate on destructuring is planned for the next release (TypeScript 1.5).

[JsonFreeman]

To your second question, yes this will be supported for in ES3/5 but only for arrays. Except that in ES5, 'for...of' works on strings too.