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{-# LANGUAGE GeneralizedNewtypeDeriving, MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, FlexibleContexts, UndecidableInstances, StandaloneDeriving #-}
module T3621 where
-- This one is ok, even though the deriving clause mentions 'a'
-- which is not a parameter of 'T'
class C a b
instance C a S
data S = MkS
newtype T = MkT S deriving( C a )
-- But this one fails, and should fail
class (Monad m) => MonadState s m | m -> s where
newtype State s a = State { runState :: s -> (a, s) }
instance Monad (State s) where {}
instance MonadState s (State s) where {}
newtype WrappedState s a = WS { runWS :: State s a }
deriving (Monad, MonadState state)
-- deriving (Monad)
deriving instance (MonadState state (State s))
=> MonadState state (WrappedState s)
-- ASSERT error
-- deriving instance (MonadState state (State s), Monad (WrappedState s))
-- => MonadState s (WrappedState s)
-- We try
-- instance MonadState state (State state a)
-- => MonadState state (WrappedState state a)
--
-- Superclass needs (Monad (WrappedState state a))
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