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

Stability | experimental |

Maintainer | libraries@haskell.org |

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 s m | m -> s where
- modify :: MonadState s m => (s -> s) -> m ()
- gets :: MonadState s m => (s -> a) -> m a

# MonadState class

class Monad m => MonadState s m | m -> s whereSource

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

*put* replaces the state inside the monad.

(Monad m, MonadTrans t, Monad (t (StateT s m))) => MonadState s (t (StateT s m)) | |

(MonadTrans t, Monad (t (State s))) => MonadState s (t (State s)) | |

MonadState s (State s) | |

(Monad m, MonadTrans t, Monad (t (StateT s m))) => MonadState s (t (StateT s m)) | |

(MonadTrans t, Monad (t (State s))) => MonadState s (t (State s)) | |

MonadState s (State s) | |

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

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

modify :: MonadState s m => (s -> s) -> 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 s m => (s -> a) -> m aSource

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