Writer - BHC Documentation

type Writer w = WriterT w Identity { } #

newtype WriterT w m a { } #

WriterT { runWriterT :: m (a, w) }

The writer monad transformer.

class Monoid w, Monad m => MonadWriter w m { } #

tellW :: w -> m ()

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

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

Class for monads that can write.

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

Run a Writer.

execWriter :: (Writer w a) -> w { } #

Run and return only the output.

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

Map both the return value and output.

<*> :: { } #

>>= :: { } #

execWriterT :: (Monad m) => (WriterT w m a) -> m w { } #

Run and return only the output.

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

Map the inner computation.

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

O(1). Produce output without a result value.

listen :: (Monad m) => (WriterT w m a) -> WriterT w m (a, w) { } #

O(1). Execute a computation and collect its output.

pass :: (Monad m) => (WriterT w m (a, w -> w)) -> WriterT w m a { } #

O(1). Execute a computation that can transform its output.

listens :: (Monad m) => (w -> b) -> (WriterT w m a) -> WriterT w m (a, b) { } #

O(1). Execute and apply a function to the output.

censor :: (Monad m) => (w -> w) -> (WriterT w m a) -> WriterT w m a { } #

O(1). Transform the output of a computation.

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

O(1). Create a writer from a result and output pair.

instance Functor m => Functor (WriterT w m) { }

instance Monoid w => MonadTrans (WriterT w) { }

instance Monoid w, MonadIO m => MonadIO (WriterT w m) { }

instance Monoid w, Monad m => MonadWriter w (WriterT w m) { }