Class.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE Safe #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 704
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE TypeFamilies #-}
#endif
#if !(MIN_VERSION_transformers(0,6,0))
{-# OPTIONS_GHC -fno-warn-deprecations #-}
#endif
#include "free-common.h"

-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Free.Class
-- Copyright   :  (C) 2008-2015 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  experimental
-- Portability :  non-portable (fundeps, MPTCs)
--
-- Monads for free.
----------------------------------------------------------------------------
module Control.Monad.Free.Class
  ( MonadFree(..)
  , liftF
  , wrapT
  ) where

import Control.Monad
import Control.Monad.Trans.Class
import Control.Monad.Trans.Reader
import qualified Control.Monad.Trans.State.Strict as Strict
import qualified Control.Monad.Trans.State.Lazy as Lazy
import qualified Control.Monad.Trans.Writer.Strict as Strict
import qualified Control.Monad.Trans.Writer.Lazy as Lazy
import qualified Control.Monad.Trans.RWS.Strict as Strict
import qualified Control.Monad.Trans.RWS.Lazy as Lazy
import Control.Monad.Trans.Cont
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Except
import Control.Monad.Trans.Identity

#if !(MIN_VERSION_transformers(0,6,0))
import Control.Monad.Trans.Error
import Control.Monad.Trans.List
#endif

#if !(MIN_VERSION_base(4,8,0))
import Control.Applicative
import Data.Monoid
#endif

-- |
-- Monads provide substitution ('fmap') and renormalization ('Control.Monad.join'):
--
-- @m '>>=' f = 'Control.Monad.join' ('fmap' f m)@
--
-- A free 'Monad' is one that does no work during the normalization step beyond simply grafting the two monadic values together.
--
-- @[]@ is not a free 'Monad' (in this sense) because @'Control.Monad.join' [[a]]@ smashes the lists flat.
--
-- On the other hand, consider:
--
-- @
-- data Tree a = Bin (Tree a) (Tree a) | Tip a
-- @
--
-- @
-- instance 'Monad' Tree where
--   'return' = Tip
--   Tip a '>>=' f = f a
--   Bin l r '>>=' f = Bin (l '>>=' f) (r '>>=' f)
-- @
--
-- This 'Monad' is the free 'Monad' of Pair:
--
-- @
-- data Pair a = Pair a a
-- @
--
-- And we could make an instance of 'MonadFree' for it directly:
--
-- @
-- instance 'MonadFree' Pair Tree where
--    'wrap' (Pair l r) = Bin l r
-- @
--
-- Or we could choose to program with @'Control.Monad.Free.Free' Pair@ instead of 'Tree'
-- and thereby avoid having to define our own 'Monad' instance.
--
-- Moreover, "Control.Monad.Free.Church" provides a 'MonadFree'
-- instance that can improve the /asymptotic/ complexity of code that
-- constructs free monads by effectively reassociating the use of
-- ('>>='). You may also want to take a look at the @kan-extensions@
-- package (<http://hackage.haskell.org/package/kan-extensions>).
--
-- See 'Control.Monad.Free.Free' for a more formal definition of the free 'Monad'
-- for a 'Functor'.
class Monad m => MonadFree f m | m -> f where
  -- | Add a layer.
  --
  -- @
  -- wrap (fmap f x) ≡ wrap (fmap return x) >>= f
  -- @
  wrap :: f (m a) -> m a
#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 704
  default wrap :: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) => f (m a) -> m a
  wrap = join . lift . wrap . fmap return
#endif

instance (Functor f, MonadFree f m) => MonadFree f (ReaderT e m) where
  wrap fm = ReaderT $ \e -> wrap $ flip runReaderT e <$> fm

instance (Functor f, MonadFree f m) => MonadFree f (Lazy.StateT s m) where
  wrap fm = Lazy.StateT $ \s -> wrap $ flip Lazy.runStateT s <$> fm

instance (Functor f, MonadFree f m) => MonadFree f (Strict.StateT s m) where
  wrap fm = Strict.StateT $ \s -> wrap $ flip Strict.runStateT s <$> fm

instance (Functor f, MonadFree f m) => MonadFree f (ContT r m) where
  wrap t = ContT $ \h -> wrap (fmap (\p -> runContT p h) t)

instance (Functor f, MonadFree f m, Monoid w) => MonadFree f (Lazy.WriterT w m) where
  wrap = Lazy.WriterT . wrap . fmap Lazy.runWriterT

instance (Functor f, MonadFree f m, Monoid w) => MonadFree f (Strict.WriterT w m) where
  wrap = Strict.WriterT . wrap . fmap Strict.runWriterT

instance (Functor f, MonadFree f m, Monoid w) => MonadFree f (Strict.RWST r w s m) where
  wrap fm = Strict.RWST $ \r s -> wrap $ fmap (\m -> Strict.runRWST m r s) fm

instance (Functor f, MonadFree f m, Monoid w) => MonadFree f (Lazy.RWST r w s m) where
  wrap fm = Lazy.RWST $ \r s -> wrap $ fmap (\m -> Lazy.runRWST m r s) fm

instance (Functor f, MonadFree f m) => MonadFree f (MaybeT m) where
  wrap = MaybeT . wrap . fmap runMaybeT

instance (Functor f, MonadFree f m) => MonadFree f (IdentityT m) where
  wrap = IdentityT . wrap . fmap runIdentityT

instance (Functor f, MonadFree f m) => MonadFree f (ExceptT e m) where
  wrap = ExceptT . wrap . fmap runExceptT

-- instance (Functor f, MonadFree f m) => MonadFree f (EitherT e m) where
--   wrap = EitherT . wrap . fmap runEitherT

#if !(MIN_VERSION_transformers(0,6,0))
instance (Functor f, MonadFree f m, Error e) => MonadFree f (ErrorT e m) where
  wrap = ErrorT . wrap . fmap runErrorT

instance (Functor f, MonadFree f m) => MonadFree f (ListT m) where
  wrap = ListT . wrap . fmap runListT
#endif

-- | A version of lift that can be used with just a Functor for f.
liftF :: (Functor f, MonadFree f m) => f a -> m a
liftF = wrap . fmap return

-- | A version of wrap for monad transformers over a free monad.
--
-- /Note:/ that this is the default implementation for 'wrap' for
-- @MonadFree f (t m)@.
wrapT :: (Functor f, MonadFree f m, MonadTrans t, Monad (t m)) => f (t m a) -> t m a
wrapT = join . lift . liftF