{-# LANGUAGE DeriveFunctor, ExistentialQuantification, FlexibleContexts, FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, ScopedTypeVariables, StandaloneDeriving, TypeOperators, UndecidableInstances #-}
module Control.Effect.Writer
( -- * Writer effect
  Writer(..)
, tell
, listen
, listens
, censor
  -- * Writer carrier
, runWriter
, execWriter
, WriterC(..)
  -- * Re-exports
, Carrier
, Member
, run
) where

import Control.Applicative (Alternative(..))
import Control.Effect.Carrier
import Control.Effect.State
import Control.Monad (MonadPlus(..))
import qualified Control.Monad.Fail as Fail
import Control.Monad.Fix
import Control.Monad.IO.Class
import Control.Monad.Trans.Class

data Writer w m k
  = Tell w (m k)
  | forall a . Listen (m a) (w -> a -> m k)
  | forall a . Censor (w -> w) (m a) (a -> m k)

deriving instance Functor m => Functor (Writer w m)

instance HFunctor (Writer w) where
  hmap :: (forall x. m x -> n x) -> Writer w m a -> Writer w n a
hmap f :: forall x. m x -> n x
f (Tell w :: w
w     k :: m a
k) = w -> n a -> Writer w n a
forall w (m :: * -> *) k. w -> m k -> Writer w m k
Tell w
w         (m a -> n a
forall x. m x -> n x
f       m a
k)
  hmap f :: forall x. m x -> n x
f (Listen   m :: m a
m k :: w -> a -> m a
k) = n a -> (w -> a -> n a) -> Writer w n a
forall w (m :: * -> *) k a. m a -> (w -> a -> m k) -> Writer w m k
Listen   (m a -> n a
forall x. m x -> n x
f m a
m) ((m a -> n a
forall x. m x -> n x
f (m a -> n a) -> (a -> m a) -> a -> n a
forall b c a. (b -> c) -> (a -> b) -> a -> c
.) ((a -> m a) -> a -> n a) -> (w -> a -> m a) -> w -> a -> n a
forall b c a. (b -> c) -> (a -> b) -> a -> c
. w -> a -> m a
k)
  hmap f :: forall x. m x -> n x
f (Censor g :: w -> w
g m :: m a
m k :: a -> m a
k) = (w -> w) -> n a -> (a -> n a) -> Writer w n a
forall w (m :: * -> *) k a.
(w -> w) -> m a -> (a -> m k) -> Writer w m k
Censor w -> w
g (m a -> n a
forall x. m x -> n x
f m a
m) (m a -> n a
forall x. m x -> n x
f     (m a -> n a) -> (a -> m a) -> a -> n a
forall b c a. (b -> c) -> (a -> b) -> a -> c
. a -> m a
k)
  {-# INLINE hmap #-}

instance Effect (Writer w) where
  handle :: f ()
-> (forall x. f (m x) -> n (f x))
-> Writer w m a
-> Writer w n (f a)
handle state :: f ()
state handler :: forall x. f (m x) -> n (f x)
handler (Tell w :: w
w     k :: m a
k) = w -> n (f a) -> Writer w n (f a)
forall w (m :: * -> *) k. w -> m k -> Writer w m k
Tell w
w                          (f (m a) -> n (f a)
forall x. f (m x) -> n (f x)
handler (m a
k m a -> f () -> f (m a)
forall (f :: * -> *) a b. Functor f => a -> f b -> f a
<$ f ()
state))
  handle state :: f ()
state handler :: forall x. f (m x) -> n (f x)
handler (Listen   m :: m a
m k :: w -> a -> m a
k) = n (f a) -> (w -> f a -> n (f a)) -> Writer w n (f a)
forall w (m :: * -> *) k a. m a -> (w -> a -> m k) -> Writer w m k
Listen   (f (m a) -> n (f a)
forall x. f (m x) -> n (f x)
handler (m a
m m a -> f () -> f (m a)
forall (f :: * -> *) a b. Functor f => a -> f b -> f a
<$ f ()
state)) ((f (m a) -> n (f a)) -> (f a -> f (m a)) -> f a -> n (f a)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap f (m a) -> n (f a)
forall x. f (m x) -> n (f x)
handler ((f a -> f (m a)) -> f a -> n (f a))
-> (w -> f a -> f (m a)) -> w -> f a -> n (f a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (a -> m a) -> f a -> f (m a)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap ((a -> m a) -> f a -> f (m a))
-> (w -> a -> m a) -> w -> f a -> f (m a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. w -> a -> m a
k)
  handle state :: f ()
state handler :: forall x. f (m x) -> n (f x)
handler (Censor f :: w -> w
f m :: m a
m k :: a -> m a
k) = (w -> w) -> n (f a) -> (f a -> n (f a)) -> Writer w n (f a)
forall w (m :: * -> *) k a.
(w -> w) -> m a -> (a -> m k) -> Writer w m k
Censor w -> w
f (f (m a) -> n (f a)
forall x. f (m x) -> n (f x)
handler (m a
m m a -> f () -> f (m a)
forall (f :: * -> *) a b. Functor f => a -> f b -> f a
<$ f ()
state)) (f (m a) -> n (f a)
forall x. f (m x) -> n (f x)
handler (f (m a) -> n (f a)) -> (f a -> f (m a)) -> f a -> n (f a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (a -> m a) -> f a -> f (m a)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap a -> m a
k)
  {-# INLINE handle #-}

-- | Write a value to the log.
--
--   prop> fst (run (runWriter (mapM_ (tell . Sum) (0 : ws)))) === foldMap Sum ws
tell :: (Member (Writer w) sig, Carrier sig m) => w -> m ()
tell :: w -> m ()
tell w :: w
w = Writer w m () -> m ()
forall (effect :: (* -> *) -> * -> *) (sig :: (* -> *) -> * -> *)
       (m :: * -> *) a.
(Member effect sig, Carrier sig m) =>
effect m a -> m a
send (w -> m () -> Writer w m ()
forall w (m :: * -> *) k. w -> m k -> Writer w m k
Tell w
w (() -> m ()
forall (f :: * -> *) a. Applicative f => a -> f a
pure ()))
{-# INLINE tell #-}

-- | Run a computation, returning the pair of its output and its result.
--
--   prop> run (runWriter (fst <$ tell (Sum a) <*> listen @(Sum Integer) (tell (Sum b)))) === (Sum a <> Sum b, Sum b)
listen :: (Member (Writer w) sig, Carrier sig m) => m a -> m (w, a)
listen :: m a -> m (w, a)
listen m :: m a
m = Writer w m (w, a) -> m (w, a)
forall (effect :: (* -> *) -> * -> *) (sig :: (* -> *) -> * -> *)
       (m :: * -> *) a.
(Member effect sig, Carrier sig m) =>
effect m a -> m a
send (m a -> (w -> a -> m (w, a)) -> Writer w m (w, a)
forall w (m :: * -> *) k a. m a -> (w -> a -> m k) -> Writer w m k
Listen m a
m (((w, a) -> m (w, a)) -> w -> a -> m (w, a)
forall a b c. ((a, b) -> c) -> a -> b -> c
curry (w, a) -> m (w, a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure))
{-# INLINE listen #-}

-- | Run a computation, applying a function to its output and returning the pair of the modified output and its result.
--
--   prop> run (runWriter (fst <$ tell (Sum a) <*> listens @(Sum Integer) (applyFun f) (tell (Sum b)))) === (Sum a <> Sum b, applyFun f (Sum b))
listens :: (Member (Writer w) sig, Carrier sig m) => (w -> b) -> m a -> m (b, a)
listens :: (w -> b) -> m a -> m (b, a)
listens f :: w -> b
f m :: m a
m = Writer w m (b, a) -> m (b, a)
forall (effect :: (* -> *) -> * -> *) (sig :: (* -> *) -> * -> *)
       (m :: * -> *) a.
(Member effect sig, Carrier sig m) =>
effect m a -> m a
send (m a -> (w -> a -> m (b, a)) -> Writer w m (b, a)
forall w (m :: * -> *) k a. m a -> (w -> a -> m k) -> Writer w m k
Listen m a
m (((b, a) -> m (b, a)) -> b -> a -> m (b, a)
forall a b c. ((a, b) -> c) -> a -> b -> c
curry (b, a) -> m (b, a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure (b -> a -> m (b, a)) -> (w -> b) -> w -> a -> m (b, a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. w -> b
f))
{-# INLINE listens #-}

-- | Run a computation, modifying its output with the passed function.
--
--   prop> run (execWriter (censor (applyFun f) (tell (Sum a)))) === applyFun f (Sum a)
--   prop> run (execWriter (tell (Sum a) *> censor (applyFun f) (tell (Sum b)) *> tell (Sum c))) === (Sum a <> applyFun f (Sum b) <> Sum c)
censor :: (Member (Writer w) sig, Carrier sig m) => (w -> w) -> m a -> m a
censor :: (w -> w) -> m a -> m a
censor f :: w -> w
f m :: m a
m = Writer w m a -> m a
forall (effect :: (* -> *) -> * -> *) (sig :: (* -> *) -> * -> *)
       (m :: * -> *) a.
(Member effect sig, Carrier sig m) =>
effect m a -> m a
send ((w -> w) -> m a -> (a -> m a) -> Writer w m a
forall w (m :: * -> *) k a.
(w -> w) -> m a -> (a -> m k) -> Writer w m k
Censor w -> w
f m a
m a -> m a
forall (f :: * -> *) a. Applicative f => a -> f a
pure)
{-# INLINE censor #-}


-- | Run a 'Writer' effect with a 'Monoid'al log, producing the final log alongside the result value.
--
--   prop> run (runWriter (tell (Sum a) *> pure b)) === (Sum a, b)
runWriter :: Monoid w => WriterC w m a -> m (w, a)
runWriter :: WriterC w m a -> m (w, a)
runWriter = w -> StateC w m a -> m (w, a)
forall s (m :: * -> *) a. s -> StateC s m a -> m (s, a)
runState w
forall a. Monoid a => a
mempty (StateC w m a -> m (w, a))
-> (WriterC w m a -> StateC w m a) -> WriterC w m a -> m (w, a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC
{-# INLINE runWriter #-}

-- | Run a 'Writer' effect with a 'Monoid'al log, producing the final log and discarding the result value.
--
--   prop> run (execWriter (tell (Sum a) *> pure b)) === Sum a
execWriter :: (Monoid w, Functor m) => WriterC w m a -> m w
execWriter :: WriterC w m a -> m w
execWriter = ((w, a) -> w) -> m (w, a) -> m w
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (w, a) -> w
forall a b. (a, b) -> a
fst (m (w, a) -> m w)
-> (WriterC w m a -> m (w, a)) -> WriterC w m a -> m w
forall b c a. (b -> c) -> (a -> b) -> a -> c
. WriterC w m a -> m (w, a)
forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter
{-# INLINE execWriter #-}


-- | A space-efficient carrier for 'Writer' effects.
--
--   This is based on a post Gabriel Gonzalez made to the Haskell mailing list: https://mail.haskell.org/pipermail/libraries/2013-March/019528.html
newtype WriterC w m a = WriterC { WriterC w m a -> StateC w m a
runWriterC :: StateC w m a }
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forall (m :: * -> *) a. Monad m => m a -> WriterC w m a
forall (t :: (* -> *) -> * -> *).
(forall (m :: * -> *) a. Monad m => m a -> t m a) -> MonadTrans t
lift :: m a -> WriterC w m a
$clift :: forall w (m :: * -> *) a. Monad m => m a -> WriterC w m a
MonadTrans)

instance (Monoid w, Carrier sig m, Effect sig) => Carrier (Writer w :+: sig) (WriterC w m) where
  eff :: (:+:) (Writer w) sig (WriterC w m) a -> WriterC w m a
eff (L (Tell w :: w
w     k :: WriterC w m a
k)) = StateC w m a -> WriterC w m a
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC (StateC w m a -> WriterC w m a) -> StateC w m a -> WriterC w m a
forall a b. (a -> b) -> a -> b
$ do
    (w -> w) -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
(s -> s) -> m ()
modify (w -> w -> w
forall a. Monoid a => a -> a -> a
`mappend` w
w)
    WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC WriterC w m a
k
  eff (L (Listen   m :: WriterC w m a
m k :: w -> a -> WriterC w m a
k)) = StateC w m a -> WriterC w m a
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC (StateC w m a -> WriterC w m a) -> StateC w m a -> WriterC w m a
forall a b. (a -> b) -> a -> b
$ do
    w
w <- StateC w m w
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
m s
get
    w -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
s -> m ()
put (w
forall a. Monoid a => a
mempty :: w)
    a
a <- WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC WriterC w m a
m
    w
w' <- StateC w m w
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
m s
get
    (w -> w) -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
(s -> s) -> m ()
modify (w -> w -> w
forall a. Monoid a => a -> a -> a
mappend (w
w :: w))
    WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC (w -> a -> WriterC w m a
k w
w' a
a)
  eff (L (Censor f :: w -> w
f m :: WriterC w m a
m k :: a -> WriterC w m a
k)) = StateC w m a -> WriterC w m a
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC (StateC w m a -> WriterC w m a) -> StateC w m a -> WriterC w m a
forall a b. (a -> b) -> a -> b
$ do
    w
w <- StateC w m w
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
m s
get
    w -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
s -> m ()
put (w
forall a. Monoid a => a
mempty :: w)
    a
a <- WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC WriterC w m a
m
    (w -> w) -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
(Member (State s) sig, Carrier sig m) =>
(s -> s) -> m ()
modify (w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w (w -> w) -> (w -> w) -> w -> w
forall b c a. (b -> c) -> (a -> b) -> a -> c
. w -> w
f)
    WriterC w m a -> StateC w m a
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC (a -> WriterC w m a
k a
a)
  eff (R other :: sig (WriterC w m) a
other)          = StateC w m a -> WriterC w m a
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC ((:+:) (State w) sig (StateC w m) a -> StateC w m a
forall (sig :: (* -> *) -> * -> *) (m :: * -> *) a.
Carrier sig m =>
sig m a -> m a
eff (sig (StateC w m) a -> (:+:) (State w) sig (StateC w m) a
forall (f :: (* -> *) -> * -> *) (g :: (* -> *) -> * -> *)
       (m :: * -> *) k.
g m k -> (:+:) f g m k
R (sig (WriterC w m) a -> sig (StateC w m) a
forall (sig :: (* -> *) -> * -> *) (f :: * -> *) (g :: * -> *) a.
(HFunctor sig, Functor f, Coercible f g) =>
sig f a -> sig g a
handleCoercible sig (WriterC w m) a
other)))
  {-# INLINE eff #-}


-- $setup
-- >>> :seti -XFlexibleContexts
-- >>> :seti -XTypeApplications
-- >>> import Test.QuickCheck
-- >>> import Control.Effect.Pure
-- >>> import Data.Semigroup (Semigroup(..), Sum(..))