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Get rid of fmapCoerce by ensuring our functors are representational #64

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66 changes: 30 additions & 36 deletions src/Control/Monad/Trans/Control.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,8 @@
, FunctionalDependencies
, FlexibleInstances
, UndecidableInstances
, MultiParamTypeClasses #-}
, MultiParamTypeClasses
, QuantifiedConstraints #-}

-- We would be Safe if Data.Coerce were.
{-# LANGUAGE Trustworthy #-}
Expand Down Expand Up @@ -142,6 +143,8 @@ import qualified Control.Monad.Trans.Writer.Strict as Strict ( WriterT(WriterT),
-- from transformers-base:
import Control.Monad.Base ( MonadBase )

class (Monad m, forall a b. (Coercible a b) => Coercible (m a) (m b)) => RepresentationalMonad m where
instance (Monad m, forall a b. (Coercible a b) => Coercible (m a) (m b)) => RepresentationalMonad m where

--------------------------------------------------------------------------------
-- MonadTransControl type class
Expand Down Expand Up @@ -247,7 +250,7 @@ class MonadTrans t => MonadTransControl t where
-- liftWith :: 'Monad' m => (('Monad' n => 'MaybeT' n b -> n ('Maybe' b)) -> m a) -> 'MaybeT' m a
-- liftWith f = 'MaybeT' ('fmap' 'Just' (f 'runMaybeT'))
-- @
liftWith :: Monad m => (Run t -> m a) -> t m a
liftWith :: RepresentationalMonad m => (Run t -> m a) -> t m a

-- | Construct a @t@ computation from the monadic state of @t@ that is
-- returned from a 'Run' function.
Expand Down Expand Up @@ -286,7 +289,7 @@ class MonadTrans t => MonadTransControl t where
-- restoreT :: ('Monad' m, 'Monoid' w) => m (a, w) -> 'WriterT' w m a
-- restoreT :: ('Monad' m, 'Monoid' w) => m (a, s, w) -> 'RWST' r w s m a
-- @
restoreT :: Monad m => m (StT t a) -> t m a
restoreT :: RepresentationalMonad m => m (StT t a) -> t m a

-- | A function that runs a transformed monad @t n@ on the monadic state that
-- was captured by 'liftWith'
Expand Down Expand Up @@ -316,16 +319,7 @@ class MonadTrans t => MonadTransControl t where
-- 'flip' 'runStateT' :: s -> Run ('StateT' s)
-- 'runMaybeT' :: Run 'MaybeT'
-- @
type Run t = forall n b. Monad n => t n b -> n (StT t b)

-------------------------------------------------------------------------------
-- fmapCoerce
-------------------------------------------------------------------------------

-- In some future this might be runtime zero-cost, but not yet.
fmapCoerce :: (Functor f, Coercible a b) => f a -> f b
fmapCoerce = fmap coerce
{-# INLINE fmapCoerce #-}
type Run t = forall n b. RepresentationalMonad n => t n b -> n (StT t b)

--------------------------------------------------------------------------------
-- Defaults for MonadTransControl
Expand Down Expand Up @@ -353,10 +347,10 @@ fmapCoerce = fmap coerce

-- | A function like 'Run' that runs a monad transformer @t@ which wraps the
-- monad transformer @t'@. This is used in 'defaultLiftWith'.
type RunDefault t t' = forall n b. Monad n => t n b -> n (StT t' b)
type RunDefault t t' = forall n b. RepresentationalMonad n => t n b -> n (StT t' b)

-- | Default definition for the 'liftWith' method.
defaultLiftWith :: (Monad m, MonadTransControl n)
defaultLiftWith :: (RepresentationalMonad m, MonadTransControl n)
=> (forall b. n m b -> t m b) -- ^ Monad constructor
-> (forall o b. t o b -> n o b) -- ^ Monad deconstructor
-> (RunDefault t n -> m a)
Expand All @@ -365,7 +359,7 @@ defaultLiftWith t unT = \f -> t $ liftWith $ \run -> f $ run . unT
{-# INLINABLE defaultLiftWith #-}

-- | Default definition for the 'restoreT' method.
defaultRestoreT :: (Monad m, MonadTransControl n)
defaultRestoreT :: (RepresentationalMonad m, MonadTransControl n)
=> (n m a -> t m a) -- ^ Monad constructor
-> m (StT n a)
-> t m a
Expand Down Expand Up @@ -395,10 +389,10 @@ defaultRestoreT t = t . restoreT

-- | A function like 'Run' that runs a monad transformer @t@ which wraps the
-- monad transformers @n@ and @n'@. This is used in 'defaultLiftWith2'.
type RunDefault2 t n n' = forall m b. (Monad m, Monad (n' m)) => t m b -> m (StT n' (StT n b))
type RunDefault2 t n n' = forall m b. (RepresentationalMonad m, RepresentationalMonad (n' m)) => t m b -> m (StT n' (StT n b))

-- | Default definition for the 'liftWith' method.
defaultLiftWith2 :: (Monad m, Monad (n' m), MonadTransControl n, MonadTransControl n')
defaultLiftWith2 :: (RepresentationalMonad m, RepresentationalMonad (n' m), MonadTransControl n, MonadTransControl n')
=> (forall b. n (n' m) b -> t m b) -- ^ Monad constructor
-> (forall o b. t o b -> n (n' o) b) -- ^ Monad deconstructor
-> (RunDefault2 t n n' -> m a)
Expand All @@ -407,7 +401,7 @@ defaultLiftWith2 t unT = \f -> t $ liftWith $ \run -> liftWith $ \run' -> f $ ru
{-# INLINABLE defaultLiftWith2 #-}

-- | Default definition for the 'restoreT' method for double 'MonadTransControl'.
defaultRestoreT2 :: (Monad m, Monad (n' m), MonadTransControl n, MonadTransControl n')
defaultRestoreT2 :: (RepresentationalMonad m, RepresentationalMonad (n' m), MonadTransControl n, MonadTransControl n')
=> (n (n' m) a -> t m a) -- ^ Monad constructor
-> m (StT n' (StT n a))
-> t m a
Expand All @@ -420,8 +414,8 @@ defaultRestoreT2 t = t . restoreT . restoreT

instance MonadTransControl IdentityT where
type StT IdentityT = Identity
liftWith f = IdentityT $ f $ fmapCoerce . runIdentityT
restoreT = IdentityT . fmapCoerce
liftWith f = IdentityT $ f $ coerce . runIdentityT
restoreT = IdentityT . coerce
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

Expand Down Expand Up @@ -455,8 +449,8 @@ instance MonadTransControl ListT where

instance MonadTransControl (ReaderT r) where
type StT (ReaderT r) = Identity
liftWith f = ReaderT $ \r -> f $ \t -> fmapCoerce $ runReaderT t r
restoreT = ReaderT . const . fmapCoerce
liftWith f = ReaderT $ \r -> f $ \t -> coerce $ runReaderT t r
restoreT = ReaderT . const . coerce
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

Expand Down Expand Up @@ -484,8 +478,8 @@ instance MonadTransControl (Strict.StateT s) where
instance Monoid w => MonadTransControl (WriterT w) where
type StT (WriterT w) = WriterStT w
liftWith f = WriterT $ fmap (\x -> (x, mempty))
(f $ fmapCoerce . runWriterT)
restoreT = WriterT . fmapCoerce
(f $ coerce . runWriterT)
restoreT = WriterT . coerce
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

Expand All @@ -495,16 +489,16 @@ newtype WriterStT w a = WriterStT { getWriterStT :: (a, w) }
instance Monoid w => MonadTransControl (Strict.WriterT w) where
type StT (Strict.WriterT w) = WriterStT w
liftWith f = Strict.WriterT $ fmap (\x -> (x, mempty))
(f $ fmapCoerce . Strict.runWriterT)
restoreT = Strict.WriterT . fmapCoerce
(f $ coerce . Strict.runWriterT)
restoreT = Strict.WriterT . coerce
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

instance Monoid w => MonadTransControl (RWST r w s) where
type StT (RWST r w s) = RwsStT w s
liftWith f = RWST $ \r s -> fmap (\x -> (x, s, mempty))
(f $ \t -> fmapCoerce $ runRWST t r s)
restoreT mSt = RWST $ \_ _ -> fmapCoerce $ mSt
(f $ \t -> coerce $ runRWST t r s)
restoreT mSt = RWST $ \_ _ -> coerce $ mSt
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

Expand All @@ -514,8 +508,8 @@ instance Monoid w => MonadTransControl (Strict.RWST r w s) where
type StT (Strict.RWST r w s) = RwsStT w s
liftWith f =
Strict.RWST $ \r s -> fmap (\x -> (x, s, mempty))
(f $ \t -> fmapCoerce $ Strict.runRWST t r s)
restoreT mSt = Strict.RWST $ \_ _ -> fmapCoerce $ mSt
(f $ \t -> coerce $ Strict.runRWST t r s)
restoreT mSt = Strict.RWST $ \_ _ -> coerce $ mSt
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}

Expand All @@ -531,7 +525,7 @@ instance Monoid w => MonadTransControl (Strict.RWST r w s) where
-- for the base monad, and @MonadTransControl T@ instances for every transformer
-- @T@. Instances for @'MonadBaseControl'@ are then simply implemented using
-- @'ComposeSt'@, @'defaultLiftBaseWith'@, @'defaultRestoreM'@.
class MonadBase b m => MonadBaseControl b m | m -> b where
class (MonadBase b m, RepresentationalMonad m, RepresentationalMonad b) => MonadBaseControl b m | m -> b where
-- | Monadic state that @m@ adds to the base monad @b@.
--
-- For all base (non-transformed) monads, @StM m a ~ a@:
Expand Down Expand Up @@ -637,7 +631,7 @@ type RunInBase m b = forall a. m a -> b (StM m a)
#define BASE(M) \
instance MonadBaseControl (M) (M) where { \
type StM (M) = Identity; \
liftBaseWith f = f fmapCoerce; \
liftBaseWith f = f coerce; \
restoreM = return . coerce; \
{-# INLINABLE liftBaseWith #-}; \
{-# INLINABLE restoreM #-}}
Expand Down Expand Up @@ -772,7 +766,7 @@ control f = liftBaseWith f >>= restoreM

-- | Lift a computation and restore the monadic state immediately:
-- @controlT f = 'liftWith' f >>= 'restoreT' . return@.
controlT :: (MonadTransControl t, Monad (t m), Monad m)
controlT :: (MonadTransControl t, RepresentationalMonad (t m), RepresentationalMonad m)
=> (Run t -> m (StT t a)) -> t m a
controlT f = liftWith f >>= restoreT . return
{-# INLINABLE controlT #-}
Expand All @@ -790,7 +784,7 @@ embed_ f = liftBaseWith $ \runInBase -> return (void . runInBase . f)
{-# INLINABLE embed_ #-}

-- | Capture the current state of a transformer
captureT :: (MonadTransControl t, Monad (t m), Monad m) => t m (StT t ())
captureT :: (MonadTransControl t, RepresentationalMonad (t m), RepresentationalMonad m) => t m (StT t ())
captureT = liftWith $ \runInM -> runInM (return ())
{-# INLINABLE captureT #-}

Expand Down Expand Up @@ -879,7 +873,7 @@ liftBaseOpDiscard f g = liftBaseWith $ \runInBase -> f $ void . runInBase . g

-- | Transform an action in @t m@ using a transformer that operates on the underlying monad @m@
liftThrough
:: (MonadTransControl t, Monad (t m), Monad m)
:: (MonadTransControl t, RepresentationalMonad (t m), RepresentationalMonad m)
=> (m (StT t a) -> m (StT t b)) -- ^
-> t m a -> t m b
liftThrough f t = do
Expand Down