{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Trans.Writer.Lazy
-- 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 lazy 'WriterT' monad transformer, which adds collection of
-- outputs (such as a count or string output) to a given monad.
--
-- This monad transformer provides only limited access to the output
-- during the computation.  For more general access, use
-- "Control.Monad.Trans.State" instead.
--
-- This version builds its output lazily; for a constant-space version
-- with almost the same interface, see "Control.Monad.Trans.Writer.CPS".
-----------------------------------------------------------------------------

module Control.Monad.Trans.Writer.Lazy (
    -- * The Writer monad
    Writer,
    writer,
    runWriter,
    execWriter,
    mapWriter,
    -- * The WriterT monad transformer
    WriterT(..),
    execWriterT,
    mapWriterT,
    -- * Writer operations
    tell,
    listen,
    listens,
    pass,
    censor,
    -- * Lifting other operations
    liftCallCC,
    liftCatch,
  ) where

import Control.Monad.IO.Class
import Control.Monad.Trans.Class
import Data.Functor.Classes
#if MIN_VERSION_base(4,12,0)
import Data.Functor.Contravariant
#endif
import Data.Functor.Identity

import Control.Applicative
import Control.Monad
#if MIN_VERSION_base(4,9,0)
import qualified Control.Monad.Fail as Fail
#endif
import Control.Monad.Fix
import Control.Monad.Signatures
#if MIN_VERSION_base(4,4,0)
import Control.Monad.Zip (MonadZip(mzipWith))
#endif
import Data.Foldable
import Data.Monoid
import Data.Traversable (Traversable(traverse))
import Prelude hiding (null, length)

-- ---------------------------------------------------------------------------
-- | A writer monad parameterized by the type @w@ of output to accumulate.
--
-- The 'return' function produces the output 'mempty', while @>>=@
-- combines the outputs of the subcomputations using 'mappend'.
type Writer w = WriterT w Identity

-- | Construct a writer computation from a (result, output) pair.
-- (The inverse of 'runWriter'.)
writer :: (Monad m) => (a, w) -> WriterT w m a
writer :: forall (m :: * -> *) a w. Monad m => (a, w) -> WriterT w m a
writer = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. Monad m => a -> m a
return
{-# INLINE writer #-}

-- | Unwrap a writer computation as a (result, output) pair.
-- (The inverse of 'writer'.)
runWriter :: Writer w a -> (a, w)
runWriter :: forall w a. Writer w a -> (a, w)
runWriter = forall a. Identity a -> a
runIdentity forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT
{-# INLINE runWriter #-}

-- | Extract the output from a writer computation.
--
-- * @'execWriter' m = 'snd' ('runWriter' m)@
execWriter :: Writer w a -> w
execWriter :: forall w a. Writer w a -> w
execWriter Writer w a
m = forall a b. (a, b) -> b
snd (forall w a. Writer w a -> (a, w)
runWriter Writer w a
m)
{-# INLINE execWriter #-}

-- | Map both the return value and output of a computation using
-- the given function.
--
-- * @'runWriter' ('mapWriter' f m) = f ('runWriter' m)@
mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
mapWriter :: forall a w b w'. ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
mapWriter (a, w) -> (b, w')
f = forall (m :: * -> *) a w (n :: * -> *) b w'.
(m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
mapWriterT (forall a. a -> Identity a
Identity forall b c a. (b -> c) -> (a -> b) -> a -> c
. (a, w) -> (b, w')
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. Identity a -> a
runIdentity)
{-# INLINE mapWriter #-}

-- ---------------------------------------------------------------------------
-- | A writer monad parameterized by:
--
--   * @w@ - the output to accumulate.
--
--   * @m@ - The inner monad.
--
-- The 'return' function produces the output 'mempty', while @>>=@
-- combines the outputs of the subcomputations using 'mappend'.
newtype WriterT w m a = WriterT { forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT :: m (a, w) }

instance (Eq w, Eq1 m) => Eq1 (WriterT w m) where
    liftEq :: forall a b.
(a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool
liftEq a -> b -> Bool
eq (WriterT m (a, w)
m1) (WriterT m (b, w)
m2) = forall (f :: * -> *) a b.
Eq1 f =>
(a -> b -> Bool) -> f a -> f b -> Bool
liftEq (forall (f :: * -> * -> *) a b c d.
Eq2 f =>
(a -> b -> Bool) -> (c -> d -> Bool) -> f a c -> f b d -> Bool
liftEq2 a -> b -> Bool
eq forall a. Eq a => a -> a -> Bool
(==)) m (a, w)
m1 m (b, w)
m2
    {-# INLINE liftEq #-}

instance (Ord w, Ord1 m) => Ord1 (WriterT w m) where
    liftCompare :: forall a b.
(a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering
liftCompare a -> b -> Ordering
comp (WriterT m (a, w)
m1) (WriterT m (b, w)
m2) =
        forall (f :: * -> *) a b.
Ord1 f =>
(a -> b -> Ordering) -> f a -> f b -> Ordering
liftCompare (forall (f :: * -> * -> *) a b c d.
Ord2 f =>
(a -> b -> Ordering)
-> (c -> d -> Ordering) -> f a c -> f b d -> Ordering
liftCompare2 a -> b -> Ordering
comp forall a. Ord a => a -> a -> Ordering
compare) m (a, w)
m1 m (b, w)
m2
    {-# INLINE liftCompare #-}

instance (Read w, Read1 m) => Read1 (WriterT w m) where
    liftReadsPrec :: forall a.
(Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a)
liftReadsPrec Int -> ReadS a
rp ReadS [a]
rl = forall a. (String -> ReadS a) -> Int -> ReadS a
readsData forall a b. (a -> b) -> a -> b
$
        forall a t.
(Int -> ReadS a) -> String -> (a -> t) -> String -> ReadS t
readsUnaryWith (forall (f :: * -> *) a.
Read1 f =>
(Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (f a)
liftReadsPrec Int -> ReadS (a, w)
rp' ReadS [(a, w)]
rl') String
"WriterT" forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT
      where
        rp' :: Int -> ReadS (a, w)
rp' = forall (f :: * -> * -> *) a b.
Read2 f =>
(Int -> ReadS a)
-> ReadS [a]
-> (Int -> ReadS b)
-> ReadS [b]
-> Int
-> ReadS (f a b)
liftReadsPrec2 Int -> ReadS a
rp ReadS [a]
rl forall a. Read a => Int -> ReadS a
readsPrec forall a. Read a => ReadS [a]
readList
        rl' :: ReadS [(a, w)]
rl' = forall (f :: * -> * -> *) a b.
Read2 f =>
(Int -> ReadS a)
-> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> ReadS [f a b]
liftReadList2 Int -> ReadS a
rp ReadS [a]
rl forall a. Read a => Int -> ReadS a
readsPrec forall a. Read a => ReadS [a]
readList

instance (Show w, Show1 m) => Show1 (WriterT w m) where
    liftShowsPrec :: forall a.
(Int -> a -> ShowS)
-> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS
liftShowsPrec Int -> a -> ShowS
sp [a] -> ShowS
sl Int
d (WriterT m (a, w)
m) =
        forall a. (Int -> a -> ShowS) -> String -> Int -> a -> ShowS
showsUnaryWith (forall (f :: * -> *) a.
Show1 f =>
(Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> f a -> ShowS
liftShowsPrec Int -> (a, w) -> ShowS
sp' [(a, w)] -> ShowS
sl') String
"WriterT" Int
d m (a, w)
m
      where
        sp' :: Int -> (a, w) -> ShowS
sp' = forall (f :: * -> * -> *) a b.
Show2 f =>
(Int -> a -> ShowS)
-> ([a] -> ShowS)
-> (Int -> b -> ShowS)
-> ([b] -> ShowS)
-> Int
-> f a b
-> ShowS
liftShowsPrec2 Int -> a -> ShowS
sp [a] -> ShowS
sl forall a. Show a => Int -> a -> ShowS
showsPrec forall a. Show a => [a] -> ShowS
showList
        sl' :: [(a, w)] -> ShowS
sl' = forall (f :: * -> * -> *) a b.
Show2 f =>
(Int -> a -> ShowS)
-> ([a] -> ShowS)
-> (Int -> b -> ShowS)
-> ([b] -> ShowS)
-> [f a b]
-> ShowS
liftShowList2 Int -> a -> ShowS
sp [a] -> ShowS
sl forall a. Show a => Int -> a -> ShowS
showsPrec forall a. Show a => [a] -> ShowS
showList

instance (Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) where == :: WriterT w m a -> WriterT w m a -> Bool
(==) = forall (f :: * -> *) a. (Eq1 f, Eq a) => f a -> f a -> Bool
eq1
instance (Ord w, Ord1 m, Ord a) => Ord (WriterT w m a) where compare :: WriterT w m a -> WriterT w m a -> Ordering
compare = forall (f :: * -> *) a. (Ord1 f, Ord a) => f a -> f a -> Ordering
compare1
instance (Read w, Read1 m, Read a) => Read (WriterT w m a) where
    readsPrec :: Int -> ReadS (WriterT w m a)
readsPrec = forall (f :: * -> *) a. (Read1 f, Read a) => Int -> ReadS (f a)
readsPrec1
instance (Show w, Show1 m, Show a) => Show (WriterT w m a) where
    showsPrec :: Int -> WriterT w m a -> ShowS
showsPrec = forall (f :: * -> *) a. (Show1 f, Show a) => Int -> f a -> ShowS
showsPrec1

-- | Extract the output from a writer computation.
--
-- * @'execWriterT' m = 'liftM' 'snd' ('runWriterT' m)@
execWriterT :: (Monad m) => WriterT w m a -> m w
execWriterT :: forall (m :: * -> *) w a. Monad m => WriterT w m a -> m w
execWriterT WriterT w m a
m = do
    ~(a
_, w
w) <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m
    forall (m :: * -> *) a. Monad m => a -> m a
return w
w
{-# INLINE execWriterT #-}

-- | Map both the return value and output of a computation using
-- the given function.
--
-- * @'runWriterT' ('mapWriterT' f m) = f ('runWriterT' m)@
mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
mapWriterT :: forall (m :: * -> *) a w (n :: * -> *) b w'.
(m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
mapWriterT m (a, w) -> n (b, w')
f WriterT w m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ m (a, w) -> n (b, w')
f (forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m)
{-# INLINE mapWriterT #-}

instance (Functor m) => Functor (WriterT w m) where
    fmap :: forall a b. (a -> b) -> WriterT w m a -> WriterT w m b
fmap a -> b
f = forall (m :: * -> *) a w (n :: * -> *) b w'.
(m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
mapWriterT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap forall a b. (a -> b) -> a -> b
$ \ ~(a
a, w
w) -> (a -> b
f a
a, w
w)
    {-# INLINE fmap #-}

instance (Foldable f) => Foldable (WriterT w f) where
    foldMap :: forall m a. Monoid m => (a -> m) -> WriterT w f a -> m
foldMap a -> m
f = forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap (a -> m
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a b. (a, b) -> a
fst) forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT
    {-# INLINE foldMap #-}
#if MIN_VERSION_base(4,8,0)
    null :: forall a. WriterT w f a -> Bool
null (WriterT f (a, w)
t) = forall (t :: * -> *) a. Foldable t => t a -> Bool
null f (a, w)
t
    length :: forall a. WriterT w f a -> Int
length (WriterT f (a, w)
t) = forall (t :: * -> *) a. Foldable t => t a -> Int
length f (a, w)
t
#endif

instance (Traversable f) => Traversable (WriterT w f) where
    traverse :: forall (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> WriterT w f a -> f (WriterT w f b)
traverse a -> f b
f = forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (t :: * -> *) (f :: * -> *) a b.
(Traversable t, Applicative f) =>
(a -> f b) -> t a -> f (t b)
traverse forall {b}. (a, b) -> f (b, b)
f' forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT where
       f' :: (a, b) -> f (b, b)
f' (a
a, b
b) = forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (\ b
c -> (b
c, b
b)) (a -> f b
f a
a)
    {-# INLINE traverse #-}

instance (Monoid w, Applicative m) => Applicative (WriterT w m) where
    pure :: forall a. a -> WriterT w m a
pure a
a  = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a. Applicative f => a -> f a
pure (a
a, forall a. Monoid a => a
mempty)
    {-# INLINE pure #-}
    WriterT w m (a -> b)
f <*> :: forall a b. WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b
<*> WriterT w m a
v = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a b c.
Applicative f =>
(a -> b -> c) -> f a -> f b -> f c
liftA2 forall {b} {t} {a}. Monoid b => (t -> a, b) -> (t, b) -> (a, b)
k (forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m (a -> b)
f) (forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
v)
      where k :: (t -> a, b) -> (t, b) -> (a, b)
k ~(t -> a
a, b
w) ~(t
b, b
w') = (t -> a
a t
b, b
w forall a. Monoid a => a -> a -> a
`mappend` b
w')
    {-# INLINE (<*>) #-}

instance (Monoid w, Alternative m) => Alternative (WriterT w m) where
    empty :: forall a. WriterT w m a
empty   = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall (f :: * -> *) a. Alternative f => f a
empty
    {-# INLINE empty #-}
    WriterT w m a
m <|> :: forall a. WriterT w m a -> WriterT w m a -> WriterT w m a
<|> WriterT w m a
n = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m forall (f :: * -> *) a. Alternative f => f a -> f a -> f a
<|> forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
n
    {-# INLINE (<|>) #-}

instance (Monoid w, Monad m) => Monad (WriterT w m) where
#if !(MIN_VERSION_base(4,8,0))
    return a = writer (a, mempty)
    {-# INLINE return #-}
#endif
    WriterT w m a
m >>= :: forall a b. WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b
>>= a -> WriterT w m b
k  = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
        ~(a
a, w
w)  <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m
        ~(b
b, w
w') <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT (a -> WriterT w m b
k a
a)
        forall (m :: * -> *) a. Monad m => a -> m a
return (b
b, w
w forall a. Monoid a => a -> a -> a
`mappend` w
w')
    {-# INLINE (>>=) #-}
#if !(MIN_VERSION_base(4,13,0))
    fail msg = WriterT $ fail msg
    {-# INLINE fail #-}
#endif

#if MIN_VERSION_base(4,9,0)
instance (Monoid w, Fail.MonadFail m) => Fail.MonadFail (WriterT w m) where
    fail :: forall a. String -> WriterT w m a
fail String
msg = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall (m :: * -> *) a. MonadFail m => String -> m a
Fail.fail String
msg
    {-# INLINE fail #-}
#endif

instance (Monoid w, MonadPlus m) => MonadPlus (WriterT w m) where
    mzero :: forall a. WriterT w m a
mzero       = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall (m :: * -> *) a. MonadPlus m => m a
mzero
    {-# INLINE mzero #-}
    WriterT w m a
m mplus :: forall a. WriterT w m a -> WriterT w m a -> WriterT w m a
`mplus` WriterT w m a
n = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m forall (m :: * -> *) a. MonadPlus m => m a -> m a -> m a
`mplus` forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
n
    {-# INLINE mplus #-}

instance (Monoid w, MonadFix m) => MonadFix (WriterT w m) where
    mfix :: forall a. (a -> WriterT w m a) -> WriterT w m a
mfix a -> WriterT w m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall (m :: * -> *) a. MonadFix m => (a -> m a) -> m a
mfix forall a b. (a -> b) -> a -> b
$ \ ~(a
a, w
_) -> forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT (a -> WriterT w m a
m a
a)
    {-# INLINE mfix #-}

instance (Monoid w) => MonadTrans (WriterT w) where
    lift :: forall (m :: * -> *) a. Monad m => m a -> WriterT w m a
lift m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
        a
a <- m a
m
        forall (m :: * -> *) a. Monad m => a -> m a
return (a
a, forall a. Monoid a => a
mempty)
    {-# INLINE lift #-}

instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where
    liftIO :: forall a. IO a -> WriterT w m a
liftIO = forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. MonadIO m => IO a -> m a
liftIO
    {-# INLINE liftIO #-}

#if MIN_VERSION_base(4,4,0)
instance (Monoid w, MonadZip m) => MonadZip (WriterT w m) where
    mzipWith :: forall a b c.
(a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c
mzipWith a -> b -> c
f (WriterT m (a, w)
x) (WriterT m (b, w)
y) = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$
        forall (m :: * -> *) a b c.
MonadZip m =>
(a -> b -> c) -> m a -> m b -> m c
mzipWith (\ ~(a
a, w
w) ~(b
b, w
w') -> (a -> b -> c
f a
a b
b, w
w forall a. Monoid a => a -> a -> a
`mappend` w
w')) m (a, w)
x m (b, w)
y
    {-# INLINE mzipWith #-}
#endif

#if MIN_VERSION_base(4,12,0)
instance Contravariant m => Contravariant (WriterT w m) where
    contramap :: forall a' a. (a' -> a) -> WriterT w m a -> WriterT w m a'
contramap a' -> a
f = forall (m :: * -> *) a w (n :: * -> *) b w'.
(m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
mapWriterT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a' a.
Contravariant f =>
(a' -> a) -> f a -> f a'
contramap forall a b. (a -> b) -> a -> b
$ \ ~(a'
a, w
w) -> (a' -> a
f a'
a, w
w)
    {-# INLINE contramap #-}
#endif

-- | @'tell' w@ is an action that produces the output @w@.
tell :: (Monad m) => w -> WriterT w m ()
tell :: forall (m :: * -> *) w. Monad m => w -> WriterT w m ()
tell w
w = forall (m :: * -> *) a w. Monad m => (a, w) -> WriterT w m a
writer ((), w
w)
{-# INLINE tell #-}

-- | @'listen' m@ is an action that executes the action @m@ and adds its
-- output to the value of the computation.
--
-- * @'runWriterT' ('listen' m) = 'liftM' (\\ (a, w) -> ((a, w), w)) ('runWriterT' m)@
listen :: (Monad m) => WriterT w m a -> WriterT w m (a, w)
listen :: forall (m :: * -> *) w a.
Monad m =>
WriterT w m a -> WriterT w m (a, w)
listen WriterT w m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
    ~(a
a, w
w) <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m
    forall (m :: * -> *) a. Monad m => a -> m a
return ((a
a, w
w), w
w)
{-# INLINE listen #-}

-- | @'listens' f m@ is an action that executes the action @m@ and adds
-- the result of applying @f@ to the output to the value of the computation.
--
-- * @'listens' f m = 'liftM' (id *** f) ('listen' m)@
--
-- * @'runWriterT' ('listens' f m) = 'liftM' (\\ (a, w) -> ((a, f w), w)) ('runWriterT' m)@
listens :: (Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
listens :: forall (m :: * -> *) w b a.
Monad m =>
(w -> b) -> WriterT w m a -> WriterT w m (a, b)
listens w -> b
f WriterT w m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
    ~(a
a, w
w) <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m
    forall (m :: * -> *) a. Monad m => a -> m a
return ((a
a, w -> b
f w
w), w
w)
{-# INLINE listens #-}

-- | @'pass' m@ is an action that executes the action @m@, which returns
-- a value and a function, and returns the value, applying the function
-- to the output.
--
-- * @'runWriterT' ('pass' m) = 'liftM' (\\ ((a, f), w) -> (a, f w)) ('runWriterT' m)@
pass :: (Monad m) => WriterT w m (a, w -> w) -> WriterT w m a
pass :: forall (m :: * -> *) w a.
Monad m =>
WriterT w m (a, w -> w) -> WriterT w m a
pass WriterT w m (a, w -> w)
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
    ~((a
a, w -> w
f), w
w) <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m (a, w -> w)
m
    forall (m :: * -> *) a. Monad m => a -> m a
return (a
a, w -> w
f w
w)
{-# INLINE pass #-}

-- | @'censor' f m@ is an action that executes the action @m@ and
-- applies the function @f@ to its output, leaving the return value
-- unchanged.
--
-- * @'censor' f m = 'pass' ('liftM' (\\ x -> (x,f)) m)@
--
-- * @'runWriterT' ('censor' f m) = 'liftM' (\\ (a, w) -> (a, f w)) ('runWriterT' m)@
censor :: (Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a
censor :: forall (m :: * -> *) w a.
Monad m =>
(w -> w) -> WriterT w m a -> WriterT w m a
censor w -> w
f WriterT w m a
m = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ do
    ~(a
a, w
w) <- forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m
    forall (m :: * -> *) a. Monad m => a -> m a
return (a
a, w -> w
f w
w)
{-# INLINE censor #-}

-- | Lift a @callCC@ operation to the new monad.
liftCallCC :: (Monoid w) => CallCC m (a,w) (b,w) -> CallCC (WriterT w m) a b
liftCallCC :: forall w (m :: * -> *) a b.
Monoid w =>
CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b
liftCallCC CallCC m (a, w) (b, w)
callCC (a -> WriterT w m b) -> WriterT w m a
f = forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$
    CallCC m (a, w) (b, w)
callCC forall a b. (a -> b) -> a -> b
$ \ (a, w) -> m (b, w)
c ->
    forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT ((a -> WriterT w m b) -> WriterT w m a
f (\ a
a -> forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ (a, w) -> m (b, w)
c (a
a, forall a. Monoid a => a
mempty)))
{-# INLINE liftCallCC #-}

-- | Lift a @catchE@ operation to the new monad.
liftCatch :: Catch e m (a,w) -> Catch e (WriterT w m) a
liftCatch :: forall e (m :: * -> *) a w.
Catch e m (a, w) -> Catch e (WriterT w m) a
liftCatch Catch e m (a, w)
catchE WriterT w m a
m e -> WriterT w m a
h =
    forall w (m :: * -> *) a. m (a, w) -> WriterT w m a
WriterT forall a b. (a -> b) -> a -> b
$ forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT WriterT w m a
m Catch e m (a, w)
`catchE` \ e
e -> forall w (m :: * -> *) a. WriterT w m a -> m (a, w)
runWriterT (e -> WriterT w m a
h e
e)
{-# INLINE liftCatch #-}