Control.Monad.Trans.Writer.Strict

Strict writer monads.

The Writer monad

type Writer w = WriterT w Identity

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

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

execWriter :: Writer w a -> w

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

The WriterT monad transformer

newtype WriterT w m a

Constructors

WriterT

Fields

runWriterT :: m (a, w)

Instances

Monoid w => MonadTrans (WriterT w)

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

Functor m => Functor (WriterT w m)

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

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

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

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

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

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

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

Writer operations

tell :: (Monoid w, Monad m) => w -> WriterT w m ()

listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w)

pass :: (Monoid w, Monad m) => WriterT w m (a, w -> w) -> WriterT w m a

listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b)

censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a

Lifting other operations

liftCallCC :: Monoid w => ((((a, w) -> m (b, w)) -> m (a, w)) -> m (a, w)) -> ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a

Lift a callCC operation to the new monad.

liftCatch :: (m (a, w) -> (e -> m (a, w)) -> m (a, w)) -> WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

Lift a catchError operation to the new monad.