Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | experimental |

Portability | non-portable (multi-param classes, functional dependencies) |

Safe Haskell | Safe |

Language | Haskell2010 |

MonadState class.

This module is inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.

- class Monad m => MonadState m where
- type StateType m

- modify :: MonadState m => (StateType m -> StateType m) -> m ()
- gets :: MonadState m => (StateType m -> a) -> m a

# MonadState class

class Monad m => MonadState m where Source #

*get* returns the state from the internals of the monad.

*put* replaces the state inside the monad.

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

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

Monad m => MonadState (StateT s m) Source # | |

Monad m => MonadState (StateT s m) Source # | |

MonadState m => MonadState (ExceptT e m) Source # | |

MonadState m => MonadState (ReaderT * r m) Source # | |

MonadState m => MonadState (ContT * r m) Source # | |

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

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

modify :: MonadState m => (StateType m -> StateType m) -> m () Source #

Monadic state transformer.

Maps an old state to a new state inside a state monad. The old state is thrown away.

Main> :t modify ((+1) :: Int -> Int) modify (...) :: (MonadState Int a) => a ()

This says that `modify (+1)`

acts over any
Monad that is a member of the `MonadState`

class,
with an `Int`

state.

gets :: MonadState m => (StateType m -> a) -> m a Source #

Gets specific component of the state, using a projection function supplied.