[Haskell-cafe] Problem with monad transformer stack

[Michael Vanier]

  Hmm, it seems like MonadState can be derived even with a non-concrete 
type, for instance:

----------

{-# 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

newtype MyMonad a b =
   MyMonad ((StateT (MyData a) (Either SomeError) b))
   deriving (Monad,
             MonadState (MyData a),
             MonadError SomeError,
             Typeable)

----------

This compiles without errors.  So it looks to me like the real problem 
was the implicit dependency between the type 'a' in MyData and the 
return type 'b' of the monad, which the deriving mechanism couldn't 
enforce if 'b' was 'a'.  I'm finding it hard to get a good conceptual 
understanding of what's really going on here.

Mike






On 10/3/10 7:03 PM, Christopher Done wrote:
> On 4 October 2010 03:40, Michael Vanier<mvanier42 at gmail.com>  wrote:
>> newtype MyMonad a =
>>   MyMonad ((StateT (MyData a) (Either SomeError) a))
>>   deriving (Monad,
>>             MonadState (MyData a),
>>             MonadError SomeError,
>>             Typeable)
> I think it's the `a'. I think it needs to be a concrete type. E.g. the
> following is OK:
>
> newtype MyMonad a =
>   MyMonad ((StateT (MyData ()) (Either SomeError) a))
>   deriving (Monad,
>             MonadState (MyData ()),
>             MonadError SomeError,
>             Typeable)
>
> But
>
> newtype MyMonad a =
>   MyMonad ((StateT (MyData ()) (Either SomeError) [a]))
>   deriving (Monad,
>             MonadState (MyData ()),
>             MonadError SomeError,
>             Typeable)
>
> is not. This reminds me of the restriction that impredicative types
> remove, but I don't think it's related.
>
>> 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.
> It seems like eta-reducing `X' or `x' is "enough", but Foo x,, i.e. a
> parametrized type with a type variable isn't "enough". I think that's
> what's going on, but I don't know why.