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 s m | m -> s where

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

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)

(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.

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

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