[Haskell-cafe] Failure deriving MonadRWS when using a type-family for the State part

[Nicolas Trangez]

The mistake might be on my side, since I expected the following to work
(but it doesn't, most likely for good reason, I didn't read any
TypeFamilies papers yet):

{-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}

module Main where

import Control.Monad.State

data S = S T
data T = T

type family F s :: *
type instance F S = T

newtype MT s m r = MT { unMT :: StateT (F s) m r }
  deriving (Monad, MonadState (F s))

-- foo :: Monad m => MT S m T
-- foo = get

{-

    Could not deduce (MonadState T (MT S m))
      arising from a use of `get'
    from the context (Monad m)
      bound by the type signature for foo :: Monad m => MT S m T
      at tf2.hs:17:1-9
    Possible fix:
      add (MonadState T (MT S m)) to the context of
        the type signature for foo :: Monad m => MT S m T
      or add an instance declaration for (MonadState T (MT S m))
    In the expression: get
    In an equation for `foo': foo = get

while GHCi says, as expected:

    λ :i MT
    newtype MT s m r = MT {unMT :: StateT (F s) m r}
      	-- Defined at tf2.hs:13:9
    instance Monad m => Monad (MT s m) -- Defined at tf2.hs:14:13
    instance Monad m => MonadState (F s) (MT s m)
      -- Defined at tf2.hs:14:20

and

    λ :t undefined :: F S
    undefined :: F S :: T

 -}

Nicolas

On Tue, 2013-05-21 at 00:08 +0200, Nicolas Trangez wrote:
> All,
> 
> The following code results in a compilation error (I tried GHC 7.4.1 & a
> 7.7.20130430 build):
> 
> {-# LANGUAGE TypeFamilies,
>              GeneralizedNewtypeDeriving,
>              StandaloneDeriving #-}
> 
> module Main where
> 
> import Control.Applicative
> import Control.Monad.RWS
> 
> data C = C
> data E = E
> 
> data S1 = S1 Int
> 
> type family I a :: *
> type instance I S1 = Int
> 
> newtype T a s m r = T { unT :: RWST C [E] (I s) m r }
>   deriving ( Functor
>            , Applicative
>            , Monad
>            , MonadReader C
>            , MonadWriter [E]
>            , MonadState (I s)
>            , MonadRWS C [E] (I s)
>            , MonadTrans
>            )
> 
> Error:
> 
>     No instance for (MonadState (I s) (T a s m))
>       arising from the 'deriving' clause of a data type declaration
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (MonadRWS C [E] (I s) (T a s m))
> 
> Commenting out the MonadRWS line from the derivings list (i.e. the line
> pointed at by the error) works as expected. I was somehow unable to get
> a suitable standalone-deriving clause working, so didn't test that.
> 
> Is this expected?
> 
> Regards,
> 
> Nicolas