Implement `for...in` syntax

[Folyd]

[JustinWoodring]

I've been looking into some possible implementations of this however the most generic solution is building iterators which are instantiated via syntactic sugar. A benefit of this solution is it abstracts a lot of logic out of the for i in 0..100 since we can treat 0..100 as shorthand for an iterator. I believe this is how Rust treats the problem.

But if we don't want to build out iterator functionality each case, for i in 0..100, the map, and the array will need independent implementations at a parser level.

Thoughts?

[Folyd]

Thanks @JustinWoodring, yes iterators is a good choice to implement it, I think it's possible to unify range (1..=100), object and array into a single Iterator AST.

[JustinWoodring]

We can approach this a couple different ways:

• Arrays expose iterator functionality, e.g. .next() which removes an element from the front of the array and leaves the rest.
  we can coerce maps and ranges into arrays. Maps are converted into arrays of key value pairs. And ranges instantiate arrays. Iterators could never be lazy in this case.

• Rust uses a trait for iterators which makes it easy to extend your own objects (this imo is the most extensible way to approach this since users can build their own iterators) but the language doesn't model interfaces or traits, so this is rather difficult. I don't think a class-level definition is a great idea for an Iterators.

• We can build a low-level iterator type that is distinct from arrays. These iterators could be lazy and or use a hidden array, e.g. a range just would need to compute its next value until it hits a stopping condition. Converting to and from these types would probably be 'private?' methods that would need to exist on arrays or maps, where they are implicitly coerced into an iterator that clones their data and stores them in a hidden array.