Control.Monad.State.Class

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.

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

(Monad (MaybeT m), MonadState m) => MonadState (MaybeT m)

(Monad (ListT m), MonadState m) => MonadState (ListT m)

(Monad (IdentityT m), MonadState m) => MonadState (IdentityT m)

(Monad (WriterT w m), Monoid w, MonadState m) => MonadState (WriterT w m)

(Monad (WriterT w m), Monoid w, MonadState m) => MonadState (WriterT w m)

(Monad (StateT s m), Monad m) => MonadState (StateT s m)

(Monad (StateT s m), Monad m) => MonadState (StateT s m)

(Monad (ReaderT r m), MonadState m) => MonadState (ReaderT r m)

(Monad (ErrorT e m), Error e, MonadState m) => MonadState (ErrorT e m)

(Monad (ContT r m), MonadState m) => MonadState (ContT r m)

(Monad (RWST r w s m), Monad m, Monoid w) => MonadState (RWST r w s m)

(Monad (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.