| Portability | non-portable (multi-param classes, functional dependencies) |
|---|---|
| Stability | experimental |
| Maintainer | libraries@haskell.org |
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 whereSource
Methods
Return the state from the internals of the monad.
Replace the state inside 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 aSource
Gets specific component of the state, using a projection function supplied.