{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE 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(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 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 :: forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter (WriterC 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 :: forall w (m :: * -> *) a.
(Monoid w, Functor m) =>
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 { forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC :: StateC w m a }
  deriving (Applicative (WriterC w m)
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-> MonadTrans (WriterC w)
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forall (t :: (* -> *) -> * -> *).
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lift :: forall (m :: * -> *) a. Monad m => m a -> WriterC w m a
$clift :: forall w (m :: * -> *) a. Monad m => m a -> WriterC w m a
MonadTrans)

instance (Monoid w, Algebra sig m) => Algebra (Writer w :+: sig) (WriterC w m) where
  alg :: forall (ctx :: * -> *) (n :: * -> *) a.
Functor ctx =>
Handler ctx n (WriterC w m)
-> (:+:) (Writer w) sig n a -> ctx () -> WriterC w m (ctx a)
alg Handler ctx n (WriterC w m)
hdl (:+:) (Writer w) sig n a
sig ctx ()
ctx = StateC w m (ctx a) -> WriterC w m (ctx a)
forall w (m :: * -> *) a. StateC w m a -> WriterC w m a
WriterC (StateC w m (ctx a) -> WriterC w m (ctx a))
-> StateC w m (ctx a) -> WriterC w m (ctx a)
forall a b. (a -> b) -> a -> b
$ case (:+:) (Writer w) sig n a
sig of
    L Writer w n a
writer -> (w -> m (w, ctx a)) -> StateC w m (ctx a)
forall s (m :: * -> *) a. (s -> m (s, a)) -> StateC s m a
StateC ((w -> m (w, ctx a)) -> StateC w m (ctx a))
-> (w -> m (w, ctx a)) -> StateC w m (ctx a)
forall a b. (a -> b) -> a -> b
$ \ w
w -> case Writer w n a
writer of
      Tell w
w'    -> do
        let !w'' :: w
w'' = w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w w
w'
        (w, ctx a) -> m (w, ctx a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure (w
w'', ctx a
ctx ()
ctx)
      Listen   n a
m -> do
        (w
w', ctx a
a) <- WriterC w m (ctx a) -> m (w, ctx a)
forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter (ctx (n a) -> WriterC w m (ctx a)
Handler ctx n (WriterC w m)
hdl (n a
m n a -> ctx () -> ctx (n a)
forall (f :: * -> *) a b. Functor f => a -> f b -> f a
<$ ctx ()
ctx))
        let !w'' :: w
w'' = w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w w
w'
        (w, ctx a) -> m (w, ctx a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure (w
w'', (,) w
w' (a -> a) -> ctx a -> ctx a
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> ctx a
a)
      Censor w -> w
f n a
m -> do
        (w
w', ctx a
a) <- WriterC w m (ctx a) -> m (w, ctx a)
forall w (m :: * -> *) a. Monoid w => WriterC w m a -> m (w, a)
runWriter (ctx (n a) -> WriterC w m (ctx a)
Handler ctx n (WriterC w m)
hdl (n a
m n a -> ctx () -> ctx (n a)
forall (f :: * -> *) a b. Functor f => a -> f b -> f a
<$ ctx ()
ctx))
        let !w'' :: w
w'' = w -> w -> w
forall a. Monoid a => a -> a -> a
mappend w
w (w -> w
f w
w')
        (w, ctx a) -> m (w, ctx a)
forall (f :: * -> *) a. Applicative f => a -> f a
pure (w
w'', ctx a
a)
    R sig n a
other  -> Handler ctx n (StateC w m)
-> (:+:) (State w) sig n a -> ctx () -> StateC w m (ctx a)
forall (sig :: (* -> *) -> * -> *) (m :: * -> *) (ctx :: * -> *)
       (n :: * -> *) a.
(Algebra sig m, Functor ctx) =>
Handler ctx n m -> sig n a -> ctx () -> m (ctx a)
alg (WriterC w m (ctx x) -> StateC w m (ctx x)
forall w (m :: * -> *) a. WriterC w m a -> StateC w m a
runWriterC (WriterC w m (ctx x) -> StateC w m (ctx x))
-> (ctx (n x) -> WriterC w m (ctx x))
-> ctx (n x)
-> StateC w m (ctx x)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. ctx (n x) -> WriterC w m (ctx x)
Handler ctx n (WriterC w m)
hdl) (sig n a -> (:+:) (State w) sig n a
forall (f :: (* -> *) -> * -> *) (g :: (* -> *) -> * -> *)
       (m :: * -> *) k.
g m k -> (:+:) f g m k
R sig n a
other) ctx ()
ctx
  {-# INLINE alg #-}