Generalized newtype deriving 6.6 vs. 6.8

Twan van Laarhoven twanvl at
Sat Nov 3 14:08:25 EDT 2007


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.


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