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-Inferred |
Language | Haskell98 |
Control.Monad.State.Class
Description
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 Source
Minimal definition is either both of get
and put
or just state
Minimal complete definition
Nothing
Methods
Return the state from the internals of the monad.
Replace the state inside the monad.
state :: (s -> (a, s)) -> m a Source
Embed a simple state action into the monad.
Instances
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) | |
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 => MonadState s (StateT s m) | |
Monad m => MonadState s (StateT s 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 () 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 a Source
Gets specific component of the state, using a projection function supplied.