Perhaps you would care to explain this then? class Runnable m n where run :: m -> n class (Monad m,Monad (t m)) => MonadT t m where up :: m a -> t m a down :: t m a -> m a instance (Monad m,MonadT t m,Monad (t m)) => Runnable (t m a) (m a) where run = down Or perhaps look at the type of the state transformer monad: newtype StateT st m a = ST { runST :: st -> m (st,a) } To me it is quite clear that the return type runST returns "m (st,a)" where m is the _base_ monad so lifting IO through the state-transformer gives: StateT st IO a and applying runST to this type gives: IO (st a)... Keean. Udo Stenzel wrote:

Keean Schupke schrieb am 20.12.04 12:13:08:

...

I thought you would get the following if you have a transformer 'M':

runM :: M IO a -> IO a

That's what I thought, too. But it's the other way 'round. Well, I can't comment on _why_ it's so counter-intuitive, but I'm pretty sure, the outermost transformer appears innermost in the final type. It all makes sense somehow, you just have to remember that transformer types are somehow written backwards.

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