Control/Monad/Trans/List.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__ >= 702
    {-# LANGUAGE Safe #-}
    #endif
    #if __GLASGOW_HASKELL__ >= 710
    {-# LANGUAGE AutoDeriveTypeable #-}
    #endif
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Monad.Trans.List
    -- Copyright   :  (c) Andy Gill 2001,
    --                (c) Oregon Graduate Institute of Science and Technology, 2001
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  R.Paterson@city.ac.uk
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- The ListT monad transformer, adding backtracking to a given monad,
    -- which must be commutative.
    -----------------------------------------------------------------------------

    module Control.Monad.Trans.List (
        -- * The ListT monad transformer
        ListT(..),
        mapListT,
        -- * Lifting other operations
        liftCallCC,
        liftCatch,
      ) where

    import Control.Monad.IO.Class
    import Control.Monad.Signatures
    import Control.Monad.Trans.Class
    import Data.Functor.Classes

    import Control.Applicative
    import Control.Monad
    #if MIN_VERSION_base(4,9,0)
    import qualified Control.Monad.Fail as Fail
    #endif
    #if MIN_VERSION_base(4,4,0)
    import Control.Monad.Zip (MonadZip(mzipWith))
    #endif
    import Data.Foldable (Foldable(foldMap))
    import Data.Traversable (Traversable(traverse))

    -- | Parameterizable list monad, with an inner monad.
    --
    -- /Note:/ this does not yield a monad unless the argument monad is commutative.
    newtype ListT m a = ListT { runListT :: m [a] }

    instance (Eq1 m) => Eq1 (ListT m) where
        liftEq eq (ListT x) (ListT y) = liftEq (liftEq eq) x y
        {-# INLINE liftEq #-}

    instance (Ord1 m) => Ord1 (ListT m) where
        liftCompare comp (ListT x) (ListT y) = liftCompare (liftCompare comp) x y
        {-# INLINE liftCompare #-}

    instance (Read1 m) => Read1 (ListT m) where
        liftReadsPrec rp rl = readsData $
            readsUnaryWith (liftReadsPrec rp' rl') "ListT" ListT
          where
            rp' = liftReadsPrec rp rl
            rl' = liftReadList rp rl

    instance (Show1 m) => Show1 (ListT m) where
        liftShowsPrec sp sl d (ListT m) =
            showsUnaryWith (liftShowsPrec sp' sl') "ListT" d m
          where
            sp' = liftShowsPrec sp sl
            sl' = liftShowList sp sl

    instance (Eq1 m, Eq a) => Eq (ListT m a) where (==) = eq1
    instance (Ord1 m, Ord a) => Ord (ListT m a) where compare = compare1
    instance (Read1 m, Read a) => Read (ListT m a) where readsPrec = readsPrec1
    instance (Show1 m, Show a) => Show (ListT m a) where showsPrec = showsPrec1

    -- | Map between 'ListT' computations.
    --
    -- * @'runListT' ('mapListT' f m) = f ('runListT' m)@
    mapListT :: (m [a] -> n [b]) -> ListT m a -> ListT n b
    mapListT f m = ListT $ f (runListT m)
    {-# INLINE mapListT #-}

    instance (Functor m) => Functor (ListT m) where
        fmap f = mapListT $ fmap $ map f
        {-# INLINE fmap #-}

    instance (Foldable f) => Foldable (ListT f) where
        foldMap f (ListT a) = foldMap (foldMap f) a
        {-# INLINE foldMap #-}

    instance (Traversable f) => Traversable (ListT f) where
        traverse f (ListT a) = ListT <$> traverse (traverse f) a
        {-# INLINE traverse #-}

    instance (Applicative m) => Applicative (ListT m) where
        pure a  = ListT $ pure [a]
        {-# INLINE pure #-}
        f <*> v = ListT $ (<*>) <$> runListT f <*> runListT v
        {-# INLINE (<*>) #-}

    instance (Applicative m) => Alternative (ListT m) where
        empty   = ListT $ pure []
        {-# INLINE empty #-}
        m <|> n = ListT $ (++) <$> runListT m <*> runListT n
        {-# INLINE (<|>) #-}

    instance (Monad m) => Monad (ListT m) where
    #if !(MIN_VERSION_base(4,8,0))
        return a = ListT $ return [a]
        {-# INLINE return #-}
    #endif
        m >>= k  = ListT $ do
            a <- runListT m
            b <- mapM (runListT . k) a
            return (concat b)
        {-# INLINE (>>=) #-}
        fail _ = ListT $ return []
        {-# INLINE fail #-}

    #if MIN_VERSION_base(4,9,0)
    instance (Monad m) => Fail.MonadFail (ListT m) where
        fail _ = ListT $ return []
        {-# INLINE fail #-}
    #endif

    instance (Monad m) => MonadPlus (ListT m) where
        mzero       = ListT $ return []
        {-# INLINE mzero #-}
        m `mplus` n = ListT $ do
            a <- runListT m
            b <- runListT n
            return (a ++ b)
        {-# INLINE mplus #-}

    instance MonadTrans ListT where
        lift m = ListT $ do
            a <- m
            return [a]
        {-# INLINE lift #-}

    instance (MonadIO m) => MonadIO (ListT m) where
        liftIO = lift . liftIO
        {-# INLINE liftIO #-}

    #if MIN_VERSION_base(4,4,0)
    instance (MonadZip m) => MonadZip (ListT m) where
        mzipWith f (ListT a) (ListT b) = ListT $ mzipWith (zipWith f) a b
        {-# INLINE mzipWith #-}
    #endif

    -- | Lift a @callCC@ operation to the new monad.
    liftCallCC :: CallCC m [a] [b] -> CallCC (ListT m) a b
    liftCallCC callCC f = ListT $
        callCC $ \ c ->
        runListT (f (\ a -> ListT $ c [a]))
    {-# INLINE liftCallCC #-}

    -- | Lift a @catchE@ operation to the new monad.
    liftCatch :: Catch e m [a] -> Catch e (ListT m) a
    liftCatch catchE m h = ListT $ runListT m
        `catchE` \ e -> runListT (h e)
    {-# INLINE liftCatch #-}