monads-fd-0.1.0.1: Monad classes, using functional dependenciesContentsIndex
Control.Monad.Writer.Class
Portabilitynon-portable (multi-param classes, functional dependencies)
Stabilityexperimental
Maintainerlibraries@haskell.org
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
class (Monoid w, Monad m) => MonadWriter w m | m -> w where
tell :: w -> m ()
listen :: m a -> m (a, w)
pass :: m (a, w -> w) -> m a
listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)
censor :: MonadWriter w m => (w -> w) -> m a -> m a
Documentation
class (Monoid w, Monad m) => MonadWriter w m | m -> w where
Methods
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.
show/hide 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)

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.

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.

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