[Haskell] Re: Implicit return values

[Ben Rudiak-Gould]

I wrote:

>   newRef   :: a -> (exists s . Ref s a, ^st s :: State)
>   readRef  :: (?st s :: State) => Ref s a -> (a, ^st s :: State)
>   writeRef :: (?st s :: State) => Ref s a -> a -> ((), ^st s :: State)
> 
> (It's not clear that readRef actually needs to return the state.)

It shouldn't, and neither should writeRef need to *take* the state. And
the type a should be moved from the ref to the state parameter. I think
this gives us statically safe polymorphic refs. And I forgot some
parentheses in newRef. I think these are the right types:

  newRef   :: a -> (exists s . (Ref s, ^st s a :: State))
  readRef  :: (?st s a :: State) => Ref s -> a
  writeRef :: Ref s -> a -> ((), ^st s a :: State)

-- Ben