Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 (c) Jeff Newbern 2003-2007 (c) Andriy Palamarchuk 2007 |
---|---|

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 |

- Computation type:
- Computations which read values from a shared environment.
- Binding strategy:
- Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment.
- Useful for:
- Maintaining variable bindings, or other shared environment.
- Zero and plus:
- None.
- Example type:
`Reader`

[(String,Value)] a

The `Reader`

monad (also called the Environment monad).
Represents a computation, which can read values from
a shared environment, pass values from function to function,
and execute sub-computations in a modified environment.
Using `Reader`

monad for such computations is often clearer and easier
than using the `State`

monad.

Inspired by the paper
*Functional Programming with Overloading and Higher-Order Polymorphism*,
Mark P Jones (http://web.cecs.pdx.edu/~mpj/)
Advanced School of Functional Programming, 1995.

- class Monad m => MonadReader r m | m -> r where
- asks :: MonadReader r m => (r -> a) -> m a

# Documentation

class Monad m => MonadReader r m | m -> r where Source #

See examples in Control.Monad.Reader.
Note, the partially applied function type `(->) r`

is a simple reader monad.
See the `instance`

declaration below.

Retrieves the monad environment.

local :: (r -> r) -> m a -> m a Source #

Executes a computation in a modified environment.

reader :: (r -> a) -> m a Source #

Retrieves a function of the current environment.

MonadReader r m => MonadReader r (MaybeT m) Source # | |

MonadReader r m => MonadReader r (ListT m) Source # | |

MonadReader r ((->) r) Source # | |

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source # | |

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source # | |

MonadReader r m => MonadReader r (StateT s m) Source # | |

MonadReader r m => MonadReader r (StateT s m) Source # | |

MonadReader r m => MonadReader r (IdentityT * m) Source # | |

MonadReader r m => MonadReader r (ExceptT e m) Source # | |

(Error e, MonadReader r m) => MonadReader r (ErrorT e m) Source # | |

MonadReader r' m => MonadReader r' (ContT * r m) Source # | |

Monad m => MonadReader r (ReaderT * r m) Source # | |

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

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

:: MonadReader r m | |

=> (r -> a) | The selector function to apply to the environment. |

-> m a |

Retrieves a function of the current environment.