Control.Monad.Writer.Class

The MonadWriter class.

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.

Documentation

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)

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: 2.2.2

Methods

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

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

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

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

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

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)

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)

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: 2.2

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)

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

(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

(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

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

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)

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)

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)