[Haskell-cafe] Rank2Types and pattern matching

Yves Parès limestrael at gmail.com
Sat Jul 3 19:24:17 EDT 2010

Hello everybody,

I'm trying to implement the type protection used by ST to prevent a monad
from returning a certain type.
There's my 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 (SomeMonad x) = runIdentity x

And when compiled, I got this error:
    Couldn't match expected type `forall s. PwetMonad s a'
           against inferred type `PwetMonad s a1'
    In the pattern: PwetMonad x
    In the definition of `runPwetMonad':
        runPwetMonad (PwetMonad x) = runIdentity x

But when I change line 10 to:
runSomeMonad x = runIdentity . unSome $ x

then it works, so obviously, the trouble is about pattern matching.
What was I doing wrong?
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