[RLEnvs] easy access of the length of an action vector (dimension of action space)

[JinraeKim]

As in JuliaRL_BC_CartPole.jl,
we often need the length of an action vector.
However, for some continuous-action environments, it seems not straightforward to obtain the length of an action vector.

How about specifying the dimension of action spaces by adding a field of each environment?
Or, providing a method for getting it.

[findmyway]

The return of action_space is undefined in RLBase. Sometimes, the result may be of a composite type, making functions like length or size meaningless.

So I prefer to extract necessary information based on the return of the action_space instead of providing a dedicated function.

[JinraeKim]

I agree with you.
However, when using IntervalSets, it may be annoying to get the dimension of action space. I mean, it seems hard to get the dimension of action space in a consistent way.

For example, one would define (d-dimensional) action space with d > 1 as, for example, action_space = Space(ClosedInterval.(_action_min, _action_max)) with vectors _action_min and _action_max.
In this case, one may be able to get the dimension as length(endpoints.(action_space(env))).
However, when d = 1, it doesn't work.

Or, do you have a good idea to resolve the issue? Please share your ideas :)

[zsunberg]

Is a vector of ClosedIntervals really the best way to represent a multidimensional action space? Then you have rand(action_space(env)) returning a ClosedInterval. Surely that is not what we want.

I think we need a dedicated multi-dimensional space type. It is probably worth creating a package for this if it does not already exist somewhere. It would be useful for POMDPs.jl also for instance

[JinraeKim]

@zsunberg Yes, I totally agree with you :)

[findmyway]

I created a new issue to track it: #268