Re: [Haskell-cafe] Composing functions with runST

I wrote:

...

Prelude Control.Monad.ST> runST (return 42) 42 Prelude Control.Monad.ST> (runST . return) 42

<interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m a1'

Brian Hulley wrote:

Hazarding a guess, I suggest it *might* be due to the fact that

forall s. ST s a

means

forall s. (ST s a)

whereas you'd need it to mean

(forall s. ST s) a

in order for it to unify with (m a).

But then why does "return 42" type successfully as forall s. (ST s a)? It needs that same unification. -Yitz