Overlapping instances

test.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FunctionalDependencies #-}

-- Let's say our project involves a number of similarly-structured reader monads.
-- Their data types might look like this:

data Conf = Conf {}
data ConfReader a = ConfReader { runConfReader :: Conf -> IO a }

-- Their instances for `Functor`, `Applicative` and `Monad` will be almost identical.
-- We only need to know how to construct and run them:

class SimpleReader m c | m -> c, c -> m where
  run  :: m a -> (c -> IO a)
  make :: (c -> IO a) -> m a

-- So let's start with `Functor`. We need `FlexibleInstances` to use a constraint
-- on an instance, and `UndecidableInstances` to allow `c` in the context but
-- not the head.

instance SimpleReader m c => Functor m where
  fmap f ma = make $ \c -> f `fmap` run ma c

-- But somehow this causes overlapping instances for Functor!

-- test.hs:24:30:
--     Overlapping instances for Functor IO arising from a use of ‘fmap’
--     Matching instances:
--       instance Functor IO -- Defined in ‘GHC.Base’
--       instance SimpleReader m c => Functor m -- Defined at test.hs:23:10
--     In the expression: f `fmap` run ma c
--     In the second argument of ‘($)’, namely ‘\ c -> f `fmap` run ma c’
--     In the expression: make $ \ c -> f `fmap` run ma c


main = return ()