mmtl-0.1: Modular Monad transformer library

Portabilitynon-portable (multi-param classes, functional dependencies)
Stabilityexperimental
Maintainerlibraries@haskell.org

Control.Monad.State.Class

Contents

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

MonadState class

class Monad m => MonadState s m | m -> s whereSource

get returns the state from the internals of the monad.

put replaces the state inside the monad.

Methods

get :: m sSource

put :: s -> m ()Source

Instances

(Monad m, MonadTrans t, Monad (t (StateT s m))) => MonadState s (t (StateT s m)) 
(MonadTrans t, Monad (t (State s))) => MonadState s (t (State s)) 
MonadState s (State s) 
(Monad m, MonadTrans t, Monad (t (StateT s m))) => MonadState s (t (StateT s m)) 
(MonadTrans t, Monad (t (State s))) => MonadState s (t (State s)) 
MonadState s (State s) 
Monad m => MonadState s (StateT s m) 
Monad m => MonadState s (StateT 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.