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
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