Re: [Haskell-cafe] Rank2Types and pattern matching

Okay, I understand better, now. But I could never have guessed it just from the GHC error message. Another question on the same code: import Control.Monad.Identity newtype SomeMonad s a = SomeMonad { unSome :: Identity a } deriving (Monad) newtype SomeType s = SomeType Int runSomeMonad :: (forall s. SomeMonad s a) -> a runSomeMonad x = runIdentity . unSome $ x foo :: SomeType s foo = runSomeMonad (return $ SomeType 42) According to what I read about ST, it should not compile because of 'foo' (hence the protection), well it does. What have I forgotten in my code?