Hello, I noticed there is a difference in generalized newtype deriving between 6.6 and 6.8. In GHC 6.4.1 the following:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Error
newtype MyMonad m a = MyMonad (m a) deriving (Monad, MonadError String)
correctly gives a derived instance MonadError String m => MonadError String (MyMonad m) The new GHC 6.8.1 complains with: No instance for (MonadError String m) arising from the 'deriving' clause of a data type declaration at DeriveTest.hs:(5,0)-(6,36) Possible fix: add an instance declaration for (MonadError String m) When deriving the instance for (MonadError String (MyMonad m)) Generalizing the instance to MonadError e m => MonadError e (MyMonad m) works in both versions. Twan