[Haskell-cafe] ST and Transformers

[J. Garrett Morris]

Hmm, that was simpler than I had imagined.  Thank you both!

 /g

On 12/22/06, Stefan O'Rear <stefanor at cox.net> wrote:
> On Fri, Dec 22, 2006 at 06:44:54PM +0100, Pepe Iborra wrote:
> > Rank-2 types seem to interact badly with (.) and ($), but my type
> > theory educated neuron doesn't know why. I think this is folklore
> > knowledge?
>
>
> You say:
> > runErrSt = (.) runST runErrorT
>
> runErrorT has type ErrorT e m a -> m (Either e a)
> runST has type (forall s. ST s a) -> a
> (.) has type (b -> c) -> (a -> b) -> (a -> c)
>
> since runST is the first argument to (.), we must unify (forall s. ST s a) with b
>
> Quoting the GHC Manual:
> > There is one place you cannot put a forall: you cannot instantiate
> > a type variable with a forall-type. So you cannot make a forall-type
> > the argument of a type constructor. So these types are illegal:
>
> I am not aware of why this restriction exists, but it is a consequence of a documented
> restriction.  (I hear rumors about limited support for "left-to-right impredicative
> instantiation" in GHC HEAD, but it's way over my head :( )
>


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