Control.Monad.Trans.Writer.Lazy

The lazy WriterT monad transformer, which adds collection of outputs (such as a count or string output) to a given monad.

This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.

This version builds its output lazily; for a strict version with the same interface, see Control.Monad.Trans.Writer.Strict.

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.

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

Construct a writer computation from a (result, output) pair. (The inverse of runWriter.)

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

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

(Show w, Show1 m) => Show1 (WriterT w m)

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

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

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

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

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

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

(Show w, Show1 m, Show a) => Show (WriterT w m a)

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)

Writer operations

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

tell w is an action that produces the output w.

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

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

• runWriterT (listen m) = liftM (\ (a, w) -> ((a, w), w)) (runWriterT m)

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

• runWriterT (listens f m) = liftM (\ (a, w) -> ((a, f w), w)) (runWriterT m)

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

• runWriterT (pass m) = liftM (\ ((a, f), w) -> (a, f w)) (runWriterT m)

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

• runWriterT (censor f m) = liftM (\ (a, w) -> (a, f w)) (runWriterT m)

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a catchE operation to the new monad.