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 (WriterType m), Monad m) => MonadWriter m where

Associated Types

type WriterType m

Methods

tell :: WriterType m -> m ()

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

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

Instances

MonadWriter m => MonadWriter (MaybeT m)

MonadWriter m => MonadWriter (IdentityT m)

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

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

MonadWriter m => MonadWriter (StateT s m)

MonadWriter m => MonadWriter (StateT s m)

MonadWriter m => MonadWriter (ReaderT r m)

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

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

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

listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)

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

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

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)

Foldable f => Foldable (WriterT w f)

Traversable f => Traversable (WriterT w f)

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

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

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

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

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

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

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

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid