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Control.Monad.State.Class

Portability | non-portable (multi-param classes, functional dependencies)
Stability | experimental
Maintainer | libraries@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://web.cecs.pdx.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 | Source

get returns the state from the internals of the monad. put replaces the state inside the monad.
Methods

get :: m s | Source

put :: s -> m () | Source

Instances

MonadState s m => MonadState s (ListT m)
MonadState s (State s)
MonadState s (State s)
(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)
MonadState s m => MonadState s (ContT r 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 () | 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.
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