[Haskell-cafe] Problem with monad transformer stack

3 Oct 2010

      I'm having a problem with a simple monad transformer stack that has me 
stumped.  Here's the sample code:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.Error
import Control.Monad.State
import Data.Typeable

data SomeError =
     Error1
   | Error2
   | ErrorFail
   deriving (Eq, Show, Typeable)

data MyData a = MyData [a]

instance Error SomeError where
   noMsg = ErrorFail

{- This works: -}
{-
newtype StateError e s a =
   StateError ((StateT s (Either e) a))
   deriving (Monad,
             MonadState s,
             MonadError e,
             Typeable)

type MyMonad a = StateError SomeError (MyData a) a
-}

{- This doesn't work: -}
newtype MyMonad a =
   MyMonad ((StateT (MyData a) (Either SomeError) a))
   deriving (Monad,
             MonadState (MyData a),
             MonadError SomeError,
             Typeable)

----------

Basically, the more abstracted (commented-out) version works, but the 
more specific one gives this error:

Weird.hs:33:12:
     Can't make a derived instance of `Monad MyMonad'
       (even with cunning newtype deriving):
       cannot eta-reduce the representation type enough
     In the newtype declaration for `MyMonad'

Weird.hs:34:12:
     Cannot eta-reduce to an instance of form
       instance (...) => MonadState (MyData a) MyMonad
     In the newtype declaration for `MyMonad'

Weird.hs:35:12:
     Can't make a derived instance of `MonadError SomeError MyMonad'
       (even with cunning newtype deriving):
       cannot eta-reduce the representation type enough
     In the newtype declaration for `MyMonad'

These error messages mean nothing to me.  What's going on?  Can the more 
specific code be made to work?  This is with ghc 6.12.3.

Thanks,

Mike

[Haskell-cafe] Problem with monad transformer stack

Michael Vanier