Control.Monad.Trans.MSF.Writer

MSFs with a Writer monadic layer.

This module contains functions to work with MSFs that include a Writer monadic layer. This includes functions to create new MSFs that include an additional layer, and functions to flatten that layer out of the MSF's transformer stack.

It is based on the _strict_ writer monad Strict, so when combining it with other modules such as mtl's, the strict version has to be included, i.e. Strict instead of Writer or Lazy.

Documentation

execWriter :: Writer w a -> w

Extract the output from a writer computation.

• execWriter m = snd (runWriter m)

execWriterT :: Monad m => WriterT w m a -> m w

Extract the output from a writer computation.

• execWriterT m = liftM snd (runWriterT m)

mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b

Map both the return value and output of a computation using the given function.

• runWriter (mapWriter f m) = f (runWriter m)

mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b

Map both the return value and output of a computation using the given function.

• runWriterT (mapWriterT f m) = f (runWriterT m)

runWriter :: Writer w a -> (a, w)

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

type Writer w = WriterT w Identity

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.

newtype WriterT w (m :: Type -> Type) a

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.

Constructors

WriterT

Fields

• runWriterT :: m (a, w)

Instances

(Monoid w, MonadSplit g m) => MonadSplit g (WriterT w m)

Defined in Control.Monad.Random.Class

Methods

getSplit :: WriterT w m g

(Monoid w, MonadBase b m) => MonadBase b (WriterT w m)

Defined in Control.Monad.Base

Methods

liftBase :: b α -> WriterT w m α

Monoid w => MonadTrans (WriterT w)

Defined in Control.Monad.Trans.Writer.Strict

Methods

lift :: Monad m => m a -> WriterT w m a

(Monoid w, MonadInterleave m) => MonadInterleave (WriterT w m)

Defined in Control.Monad.Random.Class

Methods

interleave :: WriterT w m a -> WriterT w m a

(MonadRandom m, Monoid w) => MonadRandom (WriterT w m)

Defined in Control.Monad.Random.Class

Methods

getRandomR :: Random a => (a, a) -> WriterT w m a

getRandom :: Random a => WriterT w m a

getRandomRs :: Random a => (a, a) -> WriterT w m [a]

getRandoms :: Random a => WriterT w m [a]

(Monoid w, MonadFail m) => MonadFail (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

fail :: String -> WriterT w m a

(Monoid w, MonadFix m) => MonadFix (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

mfix :: (a -> WriterT w m a) -> WriterT w m a

(Monoid w, MonadIO m) => MonadIO (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftIO :: IO a -> WriterT w m a

(Monoid w, MonadZip m) => MonadZip (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

mzip :: WriterT w m a -> WriterT w m b -> WriterT w m (a, b)

mzipWith :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c

munzip :: WriterT w m (a, b) -> (WriterT w m a, WriterT w m b)

Foldable f => Foldable (WriterT w f)

Defined in Control.Monad.Trans.Writer.Strict

Methods

fold :: Monoid m => WriterT w f m -> m

foldMap :: Monoid m => (a -> m) -> WriterT w f a -> m

foldMap' :: Monoid m => (a -> m) -> WriterT w f a -> m

foldr :: (a -> b -> b) -> b -> WriterT w f a -> b

foldr' :: (a -> b -> b) -> b -> WriterT w f a -> b

foldl :: (b -> a -> b) -> b -> WriterT w f a -> b

foldl' :: (b -> a -> b) -> b -> WriterT w f a -> b

foldr1 :: (a -> a -> a) -> WriterT w f a -> a

foldl1 :: (a -> a -> a) -> WriterT w f a -> a

toList :: WriterT w f a -> [a]

null :: WriterT w f a -> Bool

length :: WriterT w f a -> Int

elem :: Eq a => a -> WriterT w f a -> Bool

maximum :: Ord a => WriterT w f a -> a

minimum :: Ord a => WriterT w f a -> a

sum :: Num a => WriterT w f a -> a

product :: Num a => WriterT w f a -> a

(Eq w, Eq1 m) => Eq1 (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool

(Ord w, Ord1 m) => Ord1 (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering

(Read w, Read1 m) => Read1 (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (WriterT w m a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [WriterT w m a]

(Show w, Show1 m) => Show1 (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [WriterT w m a] -> ShowS

Contravariant m => Contravariant (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

contramap :: (a' -> a) -> WriterT w m a -> WriterT w m a'

(>$) :: b -> WriterT w m b -> WriterT w m a

Traversable f => Traversable (WriterT w f)

Defined in Control.Monad.Trans.Writer.Strict

Methods

traverse :: Applicative f0 => (a -> f0 b) -> WriterT w f a -> f0 (WriterT w f b)

sequenceA :: Applicative f0 => WriterT w f (f0 a) -> f0 (WriterT w f a)

mapM :: Monad m => (a -> m b) -> WriterT w f a -> m (WriterT w f b)

sequence :: Monad m => WriterT w f (m a) -> m (WriterT w f a)

(Monoid w, Alternative m) => Alternative (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

empty :: WriterT w m a

(<|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a

some :: WriterT w m a -> WriterT w m [a]

many :: WriterT w m a -> WriterT w m [a]

(Monoid w, Applicative m) => Applicative (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

pure :: a -> WriterT w m a

(<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b

liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c

(*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b

(<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a

Functor m => Functor (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

fmap :: (a -> b) -> WriterT w m a -> WriterT w m b

(<$) :: a -> WriterT w m b -> WriterT w m a

(Monoid w, Monad m) => Monad (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

(>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b

(>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b

return :: a -> WriterT w m a

(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)

Defined in Control.Monad.Trans.Writer.Strict

Methods

mzero :: WriterT w m a

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a

(Read w, Read1 m, Read a) => Read (WriterT w m a)

Defined in Control.Monad.Trans.Writer.Strict

Methods

readsPrec :: Int -> ReadS (WriterT w m a)

readList :: ReadS [WriterT w m a]

readPrec :: ReadPrec (WriterT w m a)

readListPrec :: ReadPrec [WriterT w m a]

(Show w, Show1 m, Show a) => Show (WriterT w m a)

Defined in Control.Monad.Trans.Writer.Strict

Methods

showsPrec :: Int -> WriterT w m a -> ShowS

show :: WriterT w m a -> String

showList :: [WriterT w m a] -> ShowS

(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)

Defined in Control.Monad.Trans.Writer.Strict

Methods

(==) :: WriterT w m a -> WriterT w m a -> Bool

(/=) :: WriterT w m a -> WriterT w m a -> Bool

(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)

Defined in Control.Monad.Trans.Writer.Strict

Methods

compare :: WriterT w m a -> WriterT w m a -> Ordering

(<) :: WriterT w m a -> WriterT w m a -> Bool

(<=) :: WriterT w m a -> WriterT w m a -> Bool

(>) :: WriterT w m a -> WriterT w m a -> Bool

(>=) :: WriterT w m a -> WriterT w m a -> Bool

max :: WriterT w m a -> WriterT w m a -> WriterT w m a

min :: WriterT w m a -> WriterT w m a -> WriterT w m a

writer :: forall (m :: Type -> Type) a w. Monad m => (a, w) -> WriterT w m a

Construct a writer computation from a (result, output) pair. (The inverse of runWriter.)

tell :: forall (m :: Type -> Type) w. Monad m => w -> WriterT w m ()

tell w is an action that produces the output w.

listens :: forall (m :: Type -> Type) w b a. Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b)

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)

listen :: forall (m :: Type -> Type) w a. Monad m => WriterT w m a -> WriterT w m (a, w)

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)

censor :: forall (m :: Type -> Type) w a. Monad m => (w -> w) -> WriterT w m a -> WriterT w m a

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)

Writer MSF running and wrapping

writerS :: (Functor m, Monad m, Monoid w) => MSF m a (w, b) -> MSF (WriterT w m) a b

Build an MSF in the Writer monad from one that produces the log as an extra output. This is the opposite of runWriterS.

runWriterS :: (Functor m, Monad m) => MSF (WriterT s m) a b -> MSF m a (s, b)

Build an MSF that produces the log as an extra output from one on the Writer monad. This is the opposite of writerS.