Haskell Hierarchical Libraries (mtl package)Source codeContentsIndex
Control.Monad.State.Class
Portabilitynon-portable (multi-param classes, functional dependencies)
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
MonadState class
Description

MonadState class.

This module is inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.

Synopsis
class Monad m => MonadState s m | m -> s where
get :: m s
put :: s -> m ()
modify :: MonadState s m => (s -> s) -> m ()
gets :: MonadState s m => (s -> a) -> m a
MonadState class
class Monad m => MonadState s m | m -> s where

get returns the state from the internals of the monad.

put replaces the state inside the monad.

Methods
get :: m s
put :: s -> m ()
show/hide Instances
MonadState s m => MonadState s (ListT m)
MonadState s (State s)
MonadState s (State s)
MonadState s m => MonadState s (ContT r m)
(Error e, MonadState s m) => MonadState s (ErrorT e m)
MonadState s m => MonadState s (ReaderT r m)
Monad m => MonadState s (StateT s m)
Monad m => MonadState s (StateT s m)
(Monoid w, MonadState s m) => MonadState s (WriterT w m)
(Monoid w, MonadState s m) => MonadState s (WriterT w m)
Monoid w => MonadState s (RWS r w s)
Monoid w => MonadState s (RWS r w s)
(Monoid w, Monad m) => MonadState s (RWST r w s m)
(Monoid w, Monad m) => 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.
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