Iterating over finite spaces

[wschella]

Hi!

Recently I came to the conclusion that using the FiniteSpace trait in a generic way (when building other traits and abstractions) is quite unwieldy. I think boils down to the range() function being at the same time not powerful enough, and quite strict due to the return type of ::std::ops::Range.

1. You can't implement Range<CustomType>, making it hard to implement Space for a custom type (i.e. an enum).
2. Range<T> does not imply Range<T>: Iterator<Item = T>, requiring the need to specify this bound yourself, which is usually quite awkward. E.g.: Range<State<Self>>: Iterator<Item = State<Self>>. If this is in a trait, it also requires all implementors to copy this. Iterating comfortably over all values in a space seems like quite useful for a finite space.

So I would propose doing something like this:

pub trait FiniteSpace: Space {
    type Iter: Iterator<Item = Self::Value>;

    fn iter(&self) -> Self::Iter;
}

Would you be interested in receiving PR from me applying this change?

[wschella]

Another option might be to do pub trait FiniteSpace: Space + IntoIterator<Item = <Self as Space>::Value>

[tspooner]

Hey, you make a good point about Range. Let me have a think about it and get back to you. On first thought I would rather avoid having an associated type like Iter as this adds considerable overhead. Ideally I want the traits to remain closer to the mathematics than the programming, if that makes sense.

Either way, FiniteSpace definitely needs to be revamped and perhaps this crate as a whole...

[tspooner]

Also, as always, sorry for the slow response. The NeurIPS deadline is coming up thick and fast, but rest assured I will get everything up once that's over.

[wschella]

As always, no sweat, it's not urgent, I can work around it, and my use cases are not yours. Good luck with the experimenting / writing!