Control.Monad.State.Class

monads-tf-0.1.0.2: Monad classes, using type families

Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License | BSD-style (see the file LICENSE)
Maintainer | ross@soi.city.ac.uk
Stability | experimental
Portability | non-portable (type families)
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 m where
  - type StateType m
  - get :: m (StateType m)
  - put :: StateType m -> m ()
- modify :: MonadState m => (StateType m -> StateType m) -> m ()
- gets :: MonadState m => (StateType m -> a) -> m a

# Documentation

class Monad m => MonadState m where

get returns the state from the internals of the monad.

put replaces the state inside the monad.

Associated Types

type StateType m

Methods

get :: m (StateType m)

put :: StateType m -> m ()

Instances

MonadState m => MonadState (MaybeT m) |
MonadState m => MonadState (ListT m) |
MonadState m => MonadState (IdentityT m) |
(Monoid w, MonadState m) => MonadState (WriterT w m) |
(Monoid w, MonadState m) => MonadState (WriterT w m) |
(Error e, MonadState m) => MonadState (ErrorT e m) |
Monad m => MonadState (StateT s m) |
Monad m => MonadState (StateT s m) |
MonadState m => MonadState (ReaderT r m) |
MonadState m => MonadState (ContT r m) |
(Monad m, Monoid w) => MonadState (RWST r w s m) |
(Monad m, Monoid w) => MonadState (RWST r w s m) |

modify :: MonadState m => (StateType m -> StateType m) -> 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 m => (StateType m -> a) -> m a

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