RE: Why can’t for-of be applied to iterators?

Domenic Denicola domenic at domenicdenicola.com
Tue Jun 11 06:16:09 PDT 2013


I found this comparison with C# illuminating, trying to make sense of this thread.

- It uses duck-typing for its `foreach` loops, allowing them to work with anything that has a `GetEnumerator` method.
- All of its collections do have this, by virtue of inheriting from `IEnumerable` or `IEnumerable<T>`.
- An "enumerator" has `MoveNext`, `Current`, and `Reset`.
- Generator functions return enumerables, not enumerators. Calling `GetEnumerator` on the returned enumerable gives a fresh instance, so a single call to a generator function can, by virtue of spawning multiple enumerators, allow you to create multiple invocations stepping through the generator function.

I believe this is very close to how ES works, as specced:

- `for`-`of` looks for the `@@iterator` symbol, i.e. it works on iterables which have a method for getting new iterator instances.
- Arrays and other collections all are iterable, e.g. arrays have an `@@iterator` that returns a new iterator for their values (the `values()` method does the same).
- Iterators are our friends with the `next` and optional `throw` methods.

Generator functions are a bit different in ES however:

- Each call to a generator function returns a single iterator that is also an iterable. Getting `@@iterator` returns itself. So a single call to a generator function always creates a single invocation of the generator function to step through; you cannot generate multiple invocations.

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I guess this is kind of the opposite direction Andy and Andreas have been going, but it does seem conceptually clearer to me to separate the concept of "something that can be iterated" (iterable) from "an instance of that iteration" (iterator). The former doesn't have state, usually, whereas the latter does. Hrm.


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