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

20 May 2013

      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

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

Nicolas Trangez