Control.Monad.Writer.CPS

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 :: Monoid w => Writer w a -> (a, w)

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

execWriter :: Monoid w => Writer w a -> w

Extract the output from a writer computation.

• execWriter m = snd (runWriter m)

mapWriter :: (Monoid w, Monoid w') => ((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

data 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.

Instances

MonadTrans (WriterT w)

Methods

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

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

MonadFix m => MonadFix (WriterT w m)

Methods

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

MonadFail m => MonadFail (WriterT w m)

Methods

fail :: String -> WriterT w m a

(Functor m, Monad 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

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

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

MonadIO m => MonadIO (WriterT w m)

Methods

liftIO :: IO a -> WriterT w m a

(Functor m, MonadPlus 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]

(Functor m, 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

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

Unwrap a writer computation.

execWriterT :: (Functor m, Monoid w) => WriterT w m a -> m w

Extract the output from a writer computation.

• execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (Functor n, Monoid w, Monoid w') => (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)

Orphan instances

(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