mtl.hs

{-# LANGUAGE MultiParamTypeClasses, 
             FunctionalDependencies,
             FlexibleInstances #-} --Three extensions compared to just type families
{-# LANGUAGE TypeFamilies #-}
module Mtl where
newtype ReaderT r m a = ReaderT { unReaderT :: r -> m a }

instance (Functor m) => Functor (ReaderT r m) where
  fmap f (ReaderT x) = ReaderT (\k -> fmap f (x k))

instance (Applicative m) => Applicative (ReaderT r m) where
  pure x = ReaderT (pure (pure x))
  (ReaderT f) <*> (ReaderT x) = ReaderT (\k -> f k <*> x k)

instance (Monad m) => Monad (ReaderT r m) where
  (ReaderT x) >>= f = ReaderT (\k -> x k >>= flip unReaderT k . f)

class Monad m => MonadReader m where
  type Env m
  ask :: m (Env m)

instance (Monad m) => MonadReader (ReaderT r m) where
  type Env (ReaderT r m) = r
  ask = ReaderT return

-- Fundeps version
-- class Monad m => MonadReader r m | m -> r where
--   ask :: m r

-- instance (Monad m) => MonadReader r (ReaderT r m) where
--   ask = ReaderT return