Control.Monad.Writer.Class

mtl-2.2.1: Monad classes, using functional dependencies

Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License | BSD-style (see the file LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | non-portable (multi-param classes, functional dependencies)
Safe Haskell | Safe
Language | Haskell98

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

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

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

-     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)