Portability	non-portable (multi-param classes, functional dependencies)
Stability	experimental
Maintainer	libraries@haskell.org
Safe Haskell	Safe-Infered

Control.Monad.Writer.Class

Description

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.

Synopsis

Documentation

class (Monoid w, Monad m) => MonadWriter w m | m -> w whereSource

Methods

tell :: w -> m ()Source

tell w is an action that produces the output w.

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

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 aSource

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)
MonadWriter w m => MonadWriter w (IdentityT m)
MonadWriter w m => MonadWriter w (StateT s m)
MonadWriter w m => MonadWriter w (StateT s m)
MonadWriter w m => MonadWriter w (ReaderT r m)
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)
(Monoid w, Monad m) => MonadWriter w (WriterT w m)
(Monoid w, Monad m) => MonadWriter w (WriterT w m)
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)

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

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

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)