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

20 May 2013


      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