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 | Haskell2010 |
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.
Synopsis
- 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 Source #
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 Source #
Embed a simple state action into the monad.
Instances
MonadState s m => MonadState s (MaybeT m) Source # | |
MonadState s m => MonadState s (ListT m) Source # | |
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source # | |
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source # | |
MonadState s m => MonadState s (ReaderT r m) Source # | |
MonadState s m => MonadState s (IdentityT m) Source # | |
MonadState s m => MonadState s (ExceptT e m) Source # | Since: 2.2 |
(Error e, MonadState s m) => MonadState s (ErrorT e m) Source # | |
Monad m => MonadState s (StateT s m) Source # | |
Monad m => MonadState s (StateT s m) Source # | |
MonadState s m => MonadState s (ContT r m) Source # | |
(Monad m, Monoid w) => MonadState s (RWST r w s m) Source # | |
(Monad m, Monoid w) => MonadState s (RWST r w s m) Source # | |
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.
modify' :: MonadState s m => (s -> s) -> m () Source #
A variant of modify
in which the computation is strict in the
new state.
Since: 2.2
gets :: MonadState s m => (s -> a) -> m a Source #
Gets specific component of the state, using a projection function supplied.