Hi J Garrett,
I don't see any problem in GHC 6.6. with:
> runTest c = runST (runErrorT c)
*Main> runTest (return True)
Loading package mtl-1.0 ... linking ... done.
Right True
*Main>
On the other hand, I try to define it firstly as below and got the
following error.
> runTest = runST . runErrorT
Couldn't match expected type `forall t. Test t r'
against inferred type `ErrorT e m a'
Expected type: (forall t. Test t r) -> forall s. ST s a1
Inferred type: ErrorT e m a -> m (Either e a)
In the second argument of `(.)', namely `runErrorT'
In the expression: runST . runErrorT
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?
Cheers
pepe
On 22/12/2006, at 18:26, J. Garrett Morris wrote:
Hello everyone,
I recently found myself attempting to use ST at the base of a stack of
transformers, ala:
type Test t r = ErrorT String (ST t) r
I imagined, given the type of ST, that I would need a run function
along the lines of:
runTest :: (forall t. Test t r) -> Either String r
(which was, in fact, exactly what I wanted). Unfortunately, even with
that type signature, I can't find a way to convince the compiler that
the state thread is properly universal in the call to runST. Is it
possible to write this function, or is ST (and other monads that use
its style of threading) not transformable?
Thanks,
/g
--
It is myself I have never met, whose face is pasted on the
underside of my mind.
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