Portability | non-portable (type families) |
---|---|

Stability | experimental |

Maintainer | ross@soi.city.ac.uk |

Safe Haskell | Safe-Inferred |

MonadState class.

This module is 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 => MonadState m where
- modify :: MonadState m => (StateType m -> StateType m) -> m ()
- gets :: MonadState m => (StateType m -> a) -> m a

# Documentation

class Monad m => MonadState m whereSource

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

*put* replaces the state inside the monad.

(Monad (MaybeT m), MonadState m) => MonadState (MaybeT m) | |

(Monad (ListT m), MonadState m) => MonadState (ListT m) | |

(Monad (IdentityT m), MonadState m) => MonadState (IdentityT m) | |

(Monad (WriterT w m), Monoid w, MonadState m) => MonadState (WriterT w m) | |

(Monad (WriterT w m), Monoid w, MonadState m) => MonadState (WriterT w m) | |

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

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

(Monad (ReaderT r m), MonadState m) => MonadState (ReaderT r m) | |

(Monad (ErrorT e m), Error e, MonadState m) => MonadState (ErrorT e m) | |

(Monad (ContT r m), MonadState m) => MonadState (ContT r m) | |

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

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

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 aSource

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