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Control.Monad.Trans.MSF.Writer

Contents

Writer MSF running and wrapping

Description

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.

Synopsis

newtype WriterT w (m :: Type -> Type) a = WriterT {
- runWriterT :: m (a, w)
}
type Writer w = WriterT w Identity
runWriter :: Writer w a -> (a, w)
execWriter :: Writer w a -> w
mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
execWriterT :: Monad m => WriterT w m a -> m w
mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w m a
listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
listen :: Monad m => WriterT w m a -> WriterT w m (a, w)
tell :: Monad m => w -> WriterT w m ()
writer :: Monad m => (a, w) -> WriterT w m a
writerS :: (Functor m, Monad m, Monoid w) => MSF m a (w, b) -> MSF (WriterT w m) a b
runWriterS :: (Functor m, Monad m) => MSF (WriterT s m) a b -> MSF m a (s, b)

Documentation

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)
Instance details Defined in Control.Monad.Random.Class Methods getSplit :: WriterT w m g #
(Monoid w, MonadBase b m) => MonadBase b (WriterT w m)
Instance details Defined in Control.Monad.Base Methods liftBase :: b α -> WriterT w m α #
Monoid w => MonadTrans (WriterT w)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods lift :: Monad m => m a -> WriterT w m a #
(Monoid w, Monad m) => Monad (WriterT w m)
Instance details 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 # fail :: String -> WriterT w m a #
Functor m => Functor (WriterT w m)
Instance details 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, MonadFix m) => MonadFix (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods mfix :: (a -> WriterT w m a) -> WriterT w m a #
(Monoid w, MonadFail m) => MonadFail (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods fail :: String -> WriterT w m a #
(Monoid w, Applicative m) => Applicative (WriterT w m)
Instance details 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 #
Foldable f => Foldable (WriterT w f)
Instance details 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 # 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 #
Traversable f => Traversable (WriterT w f)
Instance details 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, MonadPlus m) => MonadPlus (WriterT w m)
Instance details 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 #
(Monoid w, MonadIO m) => MonadIO (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods liftIO :: IO a -> WriterT w m a #
(MonadRandom m, Monoid w) => MonadRandom (WriterT w m)
Instance details 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, MonadInterleave m) => MonadInterleave (WriterT w m)
Instance details Defined in Control.Monad.Random.Class Methods interleave :: WriterT w m a -> WriterT w m a #
Contravariant m => Contravariant (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods contramap :: (a -> b) -> WriterT w m b -> WriterT w m a # (>$) :: b -> WriterT w m b -> WriterT w m a #
(Eq w, Eq1 m) => Eq1 (WriterT w m)
Instance details 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)
Instance details 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)
Instance details 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)
Instance details 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 #
(Monoid w, MonadZip m) => MonadZip (WriterT w m)
Instance details 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) #
(Monoid w, Alternative m) => Alternative (WriterT w m)
Instance details 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] #
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)
Instance details 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)
Instance details 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 #
(Read w, Read1 m, Read a) => Read (WriterT w m a)
Instance details 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)
Instance details 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 #

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.

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

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

execWriter :: Writer w a -> w #

Extract the output from a writer computation.

```
execWriter m = snd (runWriter 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)

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

Extract the output from a writer computation.

execWriterT m = liftM snd (runWriterT 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)

censor :: 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)

listens :: 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 :: 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)

tell :: Monad m => w -> WriterT w m () #

tell w is an action that produces the output w.

writer :: Monad m => (a, w) -> WriterT w m a #

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

`Writer` `MSF` running and wrapping

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

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) Source #

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

Documentation

Writer MSF running and wrapping

`Writer` `MSF` running and wrapping