Control.Monad.StateStack

A state monad which allows the state to be saved and restored on a stack.

Computation type:

Computations with implicit access to a read/write state, with additional operations for pushing the current state on a stack and later restoring the state from the top of the stack.

Binding strategy:

Same as for the usual state monad; the state and accompanying stack of saved states are threaded through computations.

Useful for:

Remembering state while emitting commands for some system which itself has saveable/restorable state, such as OpenGL or Cairo.

Simple example:

ghci> let p = get >>= liftIO . print
ghci> evalStateStackT (put 2 >> p >> save >> put 3 >> p >> restore >> p) 0
2
3
2

The MonadStateStack class

class MonadState s m => MonadStateStack s m where

Class of monads which support a state along with a stack for saving and restoring states.

Methods

save

Arguments

:: m ()

Save the current state on the stack

restore

Arguments

:: m ()

Restore the top state from the stack

Instances

MonadStateStack s m => MonadStateStack s (MaybeT m)

Defined in Control.Monad.StateStack

Methods

save :: MaybeT m ()

restore :: MaybeT m ()

(Monoid w, MonadStateStack s m) => MonadStateStack s (WriterT w m)

Defined in Control.Monad.StateStack

Methods

save :: WriterT w m ()

restore :: WriterT w m ()

(Monoid w, MonadStateStack s m) => MonadStateStack s (WriterT w m)

Defined in Control.Monad.StateStack

Methods

save :: WriterT w m ()

restore :: WriterT w m ()

MonadStateStack s m => MonadStateStack s (StateT s m)

Defined in Control.Monad.StateStack

Methods

save :: StateT s m ()

restore :: StateT s m ()

MonadStateStack s m => MonadStateStack s (StateT s m)

Defined in Control.Monad.StateStack

Methods

save :: StateT s m ()

restore :: StateT s m ()

MonadStateStack s m => MonadStateStack s (ReaderT r m)

Defined in Control.Monad.StateStack

Methods

save :: ReaderT r m ()

restore :: ReaderT r m ()

MonadStateStack s m => MonadStateStack s (IdentityT m)

Defined in Control.Monad.StateStack

Methods

save :: IdentityT m ()

restore :: IdentityT m ()

MonadStateStack s m => MonadStateStack s (ExceptT e m)

Defined in Control.Monad.StateStack

Methods

save :: ExceptT e m ()

restore :: ExceptT e m ()

Monad m => MonadStateStack s (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

save :: StateStackT s m ()

restore :: StateStackT s m ()

MonadStateStack s m => MonadStateStack s (ContT r m)

Defined in Control.Monad.StateStack

Methods

save :: ContT r m ()

restore :: ContT r m ()

The StateStackT transformer

newtype StateStackT s m a

A monad transformer which adds a save/restorable state to an existing monad.

Constructors

StateStackT

Fields

• unStateStackT :: StateT (s, [s]) m a

Instances

Monad m => MonadState s (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

get :: StateStackT s m s

put :: s -> StateStackT s m ()

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

Monad m => MonadStateStack s (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

save :: StateStackT s m ()

restore :: StateStackT s m ()

MonadTrans (StateStackT s)

Defined in Control.Monad.StateStack

Methods

lift :: Monad m => m a -> StateStackT s m a

Monad m => Monad (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

(>>=) :: StateStackT s m a -> (a -> StateStackT s m b) -> StateStackT s m b

(>>) :: StateStackT s m a -> StateStackT s m b -> StateStackT s m b

return :: a -> StateStackT s m a

Functor m => Functor (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

fmap :: (a -> b) -> StateStackT s m a -> StateStackT s m b

(<$) :: a -> StateStackT s m b -> StateStackT s m a

Monad m => Applicative (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

pure :: a -> StateStackT s m a

(<*>) :: StateStackT s m (a -> b) -> StateStackT s m a -> StateStackT s m b

liftA2 :: (a -> b -> c) -> StateStackT s m a -> StateStackT s m b -> StateStackT s m c

(*>) :: StateStackT s m a -> StateStackT s m b -> StateStackT s m b

(<*) :: StateStackT s m a -> StateStackT s m b -> StateStackT s m a

MonadIO m => MonadIO (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

liftIO :: IO a -> StateStackT s m a

MonadCont m => MonadCont (StateStackT s m)

Defined in Control.Monad.StateStack

Methods

callCC :: ((a -> StateStackT s m b) -> StateStackT s m a) -> StateStackT s m a

type StateStack s a = StateStackT s Identity a

Running StateStackT and StateStack computations

runStateStackT :: Monad m => StateStackT s m a -> s -> m (a, s)

Run a StateStackT computation from an initial state, resulting in a computation of the underlying monad which yields the return value and final state.

evalStateStackT :: Monad m => StateStackT s m a -> s -> m a

Like runStateStackT, but discard the final state.

execStateStackT :: Monad m => StateStackT s m a -> s -> m s

Like runStateStackT, but discard the return value and yield only the final state.

runStateStack :: StateStack s a -> s -> (a, s)

Run a StateStack computation from an initial state, resulting in a pair of the final return value and final state.

evalStateStack :: StateStack s a -> s -> a

Like runStateStack, but discard the final state.

execStateStack :: StateStack s a -> s -> s

Like runStateStack, but discard the return value and yield only the final state.

liftState :: Monad m => StateT s m a -> StateStackT s m a

StateT computations can always be lifted to StateStackT computations which do not manipulate the state stack.