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

Stability | experimental |

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

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

# Documentation

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

get :: m s

Return the state from the internals of the monad.

put :: s -> m ()

Replace the state inside the monad.

MonadState s m => MonadState s (MaybeT m) | |

MonadState s m => MonadState s (ListT m) | |

MonadState s m => MonadState s (IdentityT m) | |

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

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

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

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

MonadState s m => MonadState s (ReaderT r m) | |

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

MonadState s m => MonadState s (ContT r m) | |

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

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

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

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 a

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