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 | Haskell2010 |

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
- 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 Source #

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

embeds a simple writer action.`writer`

(a,w)

is an action that produces the output `tell`

w`w`

.

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

is an action that executes the action `listen`

m`m`

and adds
its output to the value of the computation.

pass :: m (a, w -> w) -> m a Source #

is an action that executes the action `pass`

m`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) Source # | |

MonadWriter w m => MonadWriter w (StateT s m) Source # | |

MonadWriter w m => MonadWriter w (StateT s m) Source # | |

MonadWriter w m => MonadWriter w (IdentityT m) Source # | |

MonadWriter w m => MonadWriter w (ExceptT e m) Source # | |

(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) Source # | |

(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # | |

(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # | |

MonadWriter w m => MonadWriter w (ReaderT r m) Source # | |

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

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

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