Control.Monad.Writer.Lazy

Lazy writer monads.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.

MonadWriter class

class (Monoid w, Monad m) => MonadWriter w m | m -> w where

Minimal complete definition

(writer | tell), listen, pass

Methods

writer :: (a, w) -> m a

writer (a,w) embeds a simple writer action.

tell :: w -> m ()

tell w is an action that produces the output w.

listen :: m a -> m (a, w)

listen m is an action that executes the action m and adds its output to the value of the computation.

pass :: m (a, w -> w) -> m a

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.

Instances

MonadWriter w m => MonadWriter w (MaybeT m)

Methods

writer :: (a, w) -> MaybeT m a

tell :: w -> MaybeT m ()

listen :: MaybeT m a -> MaybeT m (a, w)

pass :: MaybeT m (a, w -> w) -> MaybeT m a

Monoid w => MonadWriter w ((,) w)

NOTE: This instance is only defined for base >= 4.9.0.

Since: 2.2.2

Methods

writer :: (a, w) -> (w, a)

tell :: w -> (w, ())

listen :: (w, a) -> (w, (a, w))

pass :: (w, (a, w -> w)) -> (w, a)

MonadWriter w m => MonadWriter w (StateT s m)

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, w)

pass :: StateT s m (a, w -> w) -> StateT s m a

MonadWriter w m => MonadWriter w (StateT s m)

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, w)

pass :: StateT s m (a, w -> w) -> StateT s m a

MonadWriter w m => MonadWriter w (IdentityT m)

Methods

writer :: (a, w) -> IdentityT m a

tell :: w -> IdentityT m ()

listen :: IdentityT m a -> IdentityT m (a, w)

pass :: IdentityT m (a, w -> w) -> IdentityT m a

MonadWriter w m => MonadWriter w (ExceptT e m)

Since: 2.2

Methods

writer :: (a, w) -> ExceptT e m a

tell :: w -> ExceptT e m ()

listen :: ExceptT e m a -> ExceptT e m (a, w)

pass :: ExceptT e m (a, w -> w) -> ExceptT e m a

(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)

Methods

writer :: (a, w) -> ErrorT e m a

tell :: w -> ErrorT e m ()

listen :: ErrorT e m a -> ErrorT e m (a, w)

pass :: ErrorT e m (a, w -> w) -> ErrorT e m a

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

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, w)

pass :: WriterT w m (a, w -> w) -> WriterT w m a

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

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, w)

pass :: WriterT w m (a, w -> w) -> WriterT w m a

MonadWriter w m => MonadWriter w (ReaderT r m)

Methods

writer :: (a, w) -> ReaderT r m a

tell :: w -> ReaderT r m ()

listen :: ReaderT r m a -> ReaderT r m (a, w)

pass :: ReaderT r m (a, w -> w) -> ReaderT r m a

(Monoid w, Monad m) => MonadWriter w (RWST r w s m)

Methods

writer :: (a, w) -> RWST r w s m a

tell :: w -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, w)

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a

(Monoid w, Monad m) => MonadWriter w (RWST r w s m)

Methods

writer :: (a, w) -> RWST r w s m a

tell :: w -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, w)

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a

listens :: MonadWriter w m => (w -> b) -> m a -> 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)

censor :: MonadWriter w m => (w -> w) -> m a -> 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)

The Writer monad

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)

The WriterT monad transformer

newtype WriterT w (m :: * -> *) 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 (m (a, w))

Instances

(Monoid w, MonadError e m) => MonadError e (WriterT w m)

Methods

throwError :: e -> WriterT w m a

catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

(Monoid w, MonadState s m) => MonadState s (WriterT w m)

Methods

get :: WriterT w m s

put :: s -> WriterT w m ()

state :: (s -> (a, s)) -> WriterT w m a

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

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, w)

pass :: WriterT w m (a, w -> w) -> WriterT w m a

Monoid w => MonadTrans (WriterT w)

Methods

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

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

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)

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)

Methods

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

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

Methods

fail :: String -> WriterT w m a

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

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)

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)

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)

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

Methods

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

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

Methods

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

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

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)

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)

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

Methods

liftIO :: IO a -> WriterT w m a

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

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

Methods

mzero :: WriterT w m a

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

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

Methods

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

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

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)

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)

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)

Methods

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

show :: WriterT w m a -> String

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

runWriterT :: WriterT w m a -> m (a, w)

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)

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid