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

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 |

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 ()
- 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 #

Minimal definition is either both of `get`

and `put`

or just `state`

Return the state from the internals of the monad.

Replace the state inside the monad.

state :: (s -> (a, s)) -> m a #

Embed a simple state action into the monad.

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

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

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

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

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

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

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

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

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

MonadState s m => MonadState s (ReaderT * r 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.

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

A variant of `modify`

in which the computation is strict in the
new state.

gets :: MonadState s m => (s -> a) -> m a #

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