[Haskell-cafe] forall & ST monad

[Kim-Ee Yeoh]

Jonathan Cast-2 wrote:
> 
> Taking the `let open' syntax from `First-class
> Modules for Haskell' [1], we can say
> 
>   let open runST' = runST in
>   let
>     ref = runST' $ newSTRef 0
>     !() = runST' $ writeSTRef ref 1
>     !() = runST' $ writeSTRef ref 2
>   in runST' $ readSTRef ref
> 
> This type-checks because the let open gives us the *same* skolemized
> constant for s everywhere in the sequel.
> 

One answer, of course, is to lift the skolem type-constant into a
kind of IO-like type-monad. Sort of like getLine :: IO String,
which is still "constant" in a way, since the program 
doesn't vary.

You'd then want kind-level functions like
returnK :: * -> M * and the rest. It would be interesting
to see this made explicit in System F.

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