Recursive types

[byorgey]

Implements recursive types. Example:

tydef List a = rec l. Unit + a * l end

def nil : List a = inl () end
def cons : a -> List a -> List a = \x. \l. inr (x, l) end

def foldr : (a -> b -> b) -> b -> List a -> b = \f. \z. \xs.
  case xs
    (\_. z)
    (\c. f (fst c) (foldr f z (snd c)))
end

def map : (a -> b) -> List a -> List b = \f.
  foldr (\y. cons (f y)) nil
end

As discussed at #154, this uses the syntax rec x. F(x) to define the type x which is a solution to x = F(x). However, unlike the discussion there, I ended up going with equirecursive types (so a recursive type is equal to its unfolding). For example, as you can see above, if we have a value of type List a (defined as rec l. Unit + a * l), then it actually has the equivalent type Unit + a * List a, so we can simply do a case on it, without having to unroll first. This actually turned out to be easier to implement (and it is cooler).

There are multiple built-in functions that conceptually return a list but currently do something different (like return a fold, or take an index and return a single element, etc.) We should consider changing them to actually return lists, but that should definitely be in a separate PR.

Closes #154.

[byorgey]

I just realized I need to rule out types like rec t. t since any value may have that type, and it breaks type soundness. Should not be too hard to address, but in the meantime feel free to take a look at what's already in the PR and try it out.

[byorgey]

@xsebek I wanted to see if I could entice you back with a juicy new language feature... 😉 (also there will be another Swarm swarm June 15th: #829)

[xsebek]

@byorgey Ha, thanks, I will try this out! 👍

I noticed you needed the VSCode syntax updated and I see the rec keyword will also need to be added. 🙂

[byorgey]

This should be ready now. In theory we ought to have a device that provides the capability to use recursive types, but that will require some refactoring of the requirements analysis code so I opted to leave it for another PR: see #1898.

[nitinprakash96]

This should probably be followed up with updating the editor configurations (swarm-mode.el, swarm.vim etc)?

[byorgey]

Good catch. I will update that shortly.

[byorgey]

I updated the emacs and vim configs to add a rec keyword. I was not sure how to update the vscode config (and it seems like @xsebek is in the middle of updating it anyway).