type Writer w = WriterT w Identity Source

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while m >>= k combines the outputs of the subcomputations using mappend (also known as <>):

writer :: forall w (m :: Type -> Type) a. (Monoid w, Monad m) => (a, w) -> WriterT w m a Source

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

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

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

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

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 Source

Map both the return value and output of a computation using the given function.

• runWriter (mapWriter f m) = f (runWriter m)

data WriterT w (m :: Type -> Type) a Source

A writer monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while m >>= k combines the outputs of the subcomputations using mappend (also known as <>):

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

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

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

Unwrap a writer computation. (The inverse of writerT.)

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

Extract the output from a writer computation.

• execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b Source

Map both the return value and output of a computation using the given function.

• runWriterT (mapWriterT f m) = f (runWriterT m)

tell :: forall w (m :: Type -> Type). (Monoid w, Monad m) => w -> WriterT w m () Source

tell w is an action that produces the output w.

listen :: forall w (m :: Type -> Type) a. (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w) Source

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 :: forall w (m :: Type -> Type) b a. (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b) Source

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 :: forall w w' (m :: Type -> Type) a. (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT w' m a Source

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 :: forall w (m :: Type -> Type) a. (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a Source

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)

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

Uniform lifting of a callCC operation to the new monad. The uniformity property (see Control.Monad.Signatures) implies that the lifted callCC discards any output from the body on entering the saved continuation.

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

Lift a catchE operation to the new monad. The uniformity property (see Control.Monad.Signatures) implies that the lifted catchE discards any output from the body on entering the handler.