mtl-2.2.1: Monad classes, using functional dependencies

Copyright(c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
LicenseBSD-style (see the file LICENSE)
Maintainerlibraries@haskell.org
Stabilityexperimental
Portabilitynon-portable (multi-param classes, functional dependencies)
Safe HaskellSafe-Inferred
LanguageHaskell98

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.

Synopsis

Documentation

class Monad m => MonadState s m | m -> s where Source

Minimal definition is either both of get and put or just state

Minimal complete definition

state | get, put

Methods

get :: m s Source

Return the state from the internals of the monad.

put :: s -> m () Source

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) 
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) 
MonadState s m => MonadState s (ExceptT e 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.

modify' :: MonadState s m => (s -> s) -> m () Source

A variant of modify in which the computation is strict in the new state.

gets :: MonadState s m => (s -> a) -> m a Source

Gets specific component of the state, using a projection function supplied.