[Haskell-cafe] Why no IO transformer monad?

[Keean Schupke]

Okay but we have:

    ST s a

    and:

    runState :: State s a -> (a,s)

    runST :: (forall s . ST s a) -> a    -- notice no "s" in the return 
type:

so what about:

    runSTT :: (forall s . ST s m a) -> m a    -- no s gets out (like 
with ST not State)

After all there was the proposal to merge (IO a) and (StateT RealWorld a)

    Keean.


Udo Stenzel wrote:

Ahh, but:

runSTT :: Monad m => STT s m a -> m (a,s)


>Keean Schupke <k.schupke at imperial.ac.uk> schrieb am 20.12.04 15:17:14:
>
>  
>
>>Surely if IO is (ST RealWorld a) then (StateT RealWorld m a) is more or 
>>less the right thing?
>>    
>>
>
>Not at all, since ST and StateT are completely unrelated.  What signature would
>runSTT have?  Either it exposed the contained state (the RealWorld!):
>
>runSTT :: Monad m => STT s m a -> m (a,s)
>
>or it drops the state:
>
>runSTT :: Monad m => STT s m a -> m a
>
>The former is a bad idea, the latter is useless as it can be implemented by
>running the ordinary runST inside m.  Note that runStateT uses the former
>signature and would expose RealWorld, if it could be instantiated at RealWorld.
>
>
>Udo.
>
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