{-# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, TypeOperators, UndecidableInstances #-}

{- | A carrier for 'Writer' effects. This carrier performs its append operations strictly and thus avoids the space leaks inherent in lazy writer monads. These appends are left-associative; as such, @[]@ is a poor choice of monoid for computations that entail many calls to 'tell'. The [Seq](http://hackage.haskell.org/package/containersdocs/Data-Sequence.html) or [DList](http://hackage.haskell.org/package/dlist) monoids may be a superior choice.

This implementation is based on a post Gabriel Gonzalez made to the Haskell mailing list: <https://mail.haskell.org/pipermail/libraries/2013-March/019528.html>

@since 1.0.0.0
-}

module Control.Carrier.Writer.Strict
( -- * Writer carrier
  runWriter
, execWriter
, WriterC(..)
  -- * Writer effect
, module Control.Effect.Writer
) where

import Control.Algebra
import Control.Applicative (Alternative(..))
import Control.Carrier.State.Strict
import Control.Effect.Writer
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

-- | Run a 'Writer' effect with a 'Monoid'al log, producing the final log alongside the result value.
--
-- @
-- 'runWriter' ('tell' w) = 'pure' (w, ())
-- @
-- @
-- 'runWriter' ('pure' a) = 'pure' ('mempty', a)
-- @
runWriter :: Monoid w => WriterC w m a -> m (w, a)
runWriter :: WriterC w m a -> m (w, a)
runWriter (WriterC m :: StateC w m a
m) = 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
{-# INLINE runWriter #-}

-- | Run a 'Writer' effect with a 'Monoid'al log, producing the final log and discarding the result value.
--
-- @
-- 'execWriter' m = 'fmap' 'fst' ('runWriter' m)
-- @
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, implemented atop "Control.Carrier.State.Strict".
--
-- @since 1.0.0.0
newtype WriterC w m a = WriterC (StateC w m a)
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instance (Monoid w, Algebra sig m, Effect sig) => Algebra (Writer w :+: sig) (WriterC w m) where
  alg :: (:+:) (Writer w) sig (WriterC w m) a -> WriterC w m a
alg (L (Tell w :: w
w     k :: WriterC w m a
k)) = StateC w m () -> WriterC w m ()
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC ((w -> w) -> StateC w m ()
forall s (sig :: (* -> *) -> * -> *) (m :: * -> *).
Has (State s) sig m =>
(s -> s) -> m ()
modify (w -> w -> w
forall a. Monoid a => a -> a -> a
`mappend` w
w)) WriterC w m () -> WriterC w m a -> WriterC w m a
forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
>> WriterC w m a
k
  alg (L (Listen   m :: WriterC w m a
m k :: w -> a -> WriterC w m a
k)) = StateC w m (w, a) -> WriterC w m (w, a)
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC ((w -> m (w, (w, a))) -> StateC w m (w, a)
forall s (m :: * -> *) a. (s -> m (s, a)) -> StateC s m a
StateC (\ w :: w
w -> do
    (w' :: w
w', a :: a
a) <- WriterC w m a -> m (w, a)
forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter WriterC w m a
m
    let w'' :: w
w'' = w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w w
w'
    w
w'' w -> m (w, (w, a)) -> m (w, (w, a))
forall a b. a -> b -> b
`seq` (w, (w, a)) -> m (w, (w, a))
forall (f :: * -> *) a. Applicative f => a -> f a
pure (w
w'', (w
w', a
a))))
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forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= (w -> a -> WriterC w m a) -> (w, a) -> WriterC w m a
forall a b c. (a -> b -> c) -> (a, b) -> c
uncurry w -> a -> WriterC w m a
k
  alg (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 ((w -> m (w, a)) -> StateC w m a
forall s (m :: * -> *) a. (s -> m (s, a)) -> StateC s m a
StateC (\ w :: w
w -> do
    (w' :: w
w', a :: a
a) <- WriterC w m a -> m (w, a)
forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter WriterC w m a
m
    let w'' :: w
w'' = w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w (w -> w
f w
w')
    w
w'' w -> m (w, a) -> m (w, a)
forall a b. a -> b -> b
`seq` (w, a) -> m (w, a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure (w
w'', a
a)))
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forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= a -> WriterC w m a
k
  alg (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.
Algebra sig m =>
sig m a -> m a
alg (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 alg #-}