Control.Monad.State.Class

mtl-2.2.1: Monad classes, using functional dependencies

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 | Haskell98

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

- class Monad m => MonadState s m | m -> s where
  - get :: m s
  - put :: s -> m ()
  - state :: (s -> (a, s)) -> m a
- 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

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

Minimal complete definition

state | get, put

Methods

get :: m s

Return the state from the internals of the monad.

put :: s -> m ()

Replace the state inside the monad.

state :: (s -> (a, s)) -> m a

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 ()

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 ()

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

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

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