Writer

Writer

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)

WriterT

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)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

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

Monad m => Monad (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS.Internal

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)

Defined in Control.Monad.Trans.Writer.CPS.Internal

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)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

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

MonadFail m => MonadFail (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

fail :: String -> WriterT w m a

(Functor m, Monad m) => Applicative (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

pure :: a -> WriterT w m a

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

liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c

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

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

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

Defined in Control.Monad.Trans.Writer.CPS.Internal

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)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

mzero :: WriterT w m a

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a

MonadIO m => MonadIO (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS.Internal

Methods

liftIO :: IO a -> WriterT w m a

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

The WriterT constructor is deliberately not exported in the CPS module to avoid exposing the hidden state w. writerT provides a safe way to construct a WriterT with the same api as the original WriterT.

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

Unwrap a writer computation.

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

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

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

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

MonadWriter

class (Monoid w, Monad m) => MonadWriter w (m :: * -> *) | m -> w where

Minimal complete definition

(writer | tell), listen, pass

Methods

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

writer (a,w) embeds a simple writer action.

tell :: w -> m ()

tell w is an action that produces the output w.

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

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

pass :: m (a, w -> 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.

Instances

MonadWriter w m => MonadWriter w (MaybeT m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> MaybeT m a

tell :: w -> MaybeT m ()

listen :: MaybeT m a -> MaybeT m (a, w)

pass :: MaybeT m (a, w -> w) -> MaybeT m a

Monoid w => MonadWriter w ((,) w)

NOTE: This instance is only defined for base >= 4.9.0.

Since: mtl-2.2.2

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> (w, a)

tell :: w -> (w, ())

listen :: (w, a) -> (w, (a, w))

pass :: (w, (a, w -> w)) -> (w, a)

(Functor m, MonadWriter e m) => MonadWriter e (Free m)

Defined in Control.Monad.Free

Methods

writer :: (a, e) -> Free m a

tell :: e -> Free m ()

listen :: Free m a -> Free m (a, e)

pass :: Free m (a, e -> e) -> Free m a

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

Defined in Control.Monad.Writer.Class

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

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

Defined in Control.Monad.Writer.Class

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

MonadWriter w m => MonadWriter w (StateT s m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, w)

pass :: StateT s m (a, w -> w) -> StateT s m a

MonadWriter w m => MonadWriter w (StateT s m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, w)

pass :: StateT s m (a, w -> w) -> StateT s m a

MonadWriter w m => MonadWriter w (IdentityT m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> IdentityT m a

tell :: w -> IdentityT m ()

listen :: IdentityT m a -> IdentityT m (a, w)

pass :: IdentityT m (a, w -> w) -> IdentityT m a

MonadWriter w m => MonadWriter w (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ExceptT e m a

tell :: w -> ExceptT e m ()

listen :: ExceptT e m a -> ExceptT e m (a, w)

pass :: ExceptT e m (a, w -> w) -> ExceptT e m a

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

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ErrorT e m a

tell :: w -> ErrorT e m ()

listen :: ErrorT e m a -> ErrorT e m (a, w)

pass :: ErrorT e m (a, w -> w) -> ErrorT e m a

(Functor f, MonadWriter w m) => MonadWriter w (FreeT f m)

Defined in Control.Monad.Trans.Free

Methods

writer :: (a, w) -> FreeT f m a

tell :: w -> FreeT f m ()

listen :: FreeT f m a -> FreeT f m (a, w)

pass :: FreeT f m (a, w -> w) -> FreeT f m a

MonadWriter w m => MonadWriter w (ReaderT r m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ReaderT r m a

tell :: w -> ReaderT r m ()

listen :: ReaderT r m a -> ReaderT r m (a, w)

pass :: ReaderT r m (a, w -> w) -> ReaderT r m a

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

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> RWST r w s m a

tell :: w -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, w)

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a

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

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> RWST r w s m a

tell :: w -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, w)

pass :: RWST r w s m (a, w -> w) -> RWST r w s m a

listens :: MonadWriter w m => (w -> b) -> m a -> 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)

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