Control.Monad.Representable.State

A generalized State monad, parameterized by a Representable functor. The representation of that functor serves as the state.

Documentation

type State g = StateT g Identity

A memoized state monad parameterized by a representable functor g, where the representatation of g, Key g is the state to carry.

The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

runState

Arguments

:: Indexable g

=> State g a

state-passing computation to execute

-> Key g

initial state

-> (a, Key g)

return value and final state

Unwrap a state monad computation as a function. (The inverse of state.)

evalState

Arguments

:: Indexable g

=> State g a

state-passing computation to execute

-> Key g

initial value

-> a

return value of the state computation

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

• evalState m s = fst (runState m s)

execState

Arguments

:: Indexable g

=> State g a

state-passing computation to execute

-> Key g

initial value

-> Key g

final state

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

• execState m s = snd (runState m s)

mapState :: Functor g => ((a, Key g) -> (b, Key g)) -> State g a -> State g b

Map both the return value and final state of a computation using the given function.

• runState (mapState f m) = f . runState m

newtype StateT g m a

A state transformer monad parameterized by:

• g - A representable functor used to memoize results for a state Key g
• m - The inner monad.

The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

Constructors

StateT

Fields

getStateT :: g (m (a, Key g))

Instances

(Monad (StateT g m), Functor f, Representable g, MonadFree f m) => MonadFree f (StateT g m)

(Monad (StateT g m), Representable g, MonadReader e m) => MonadReader e (StateT g m)

(Monad (StateT g m), Representable g, Monad m, ~ * (Key g) s) => MonadState s (StateT g m)

(Monoid w, Monad (StateT g m), Representable g, MonadWriter w m) => MonadWriter w (StateT g m)

Representable f => MonadTrans (StateT f)

(MonadTrans (StateT f), Representable f) => BindTrans (StateT f)

(Representable g, Monad m) => Monad (StateT g m)

(Functor g, Functor m) => Functor (StateT g m)

(Functor (StateT g m), Representable g, Functor m, Monad m) => Applicative (StateT g m)

(Monad (StateT g m), Representable g, MonadCont m) => MonadCont (StateT g m)

(Functor (StateT g m), Functor g, Indexable g, Bind m) => Apply (StateT g m)

(Apply (StateT g m), Functor g, Indexable g, Bind m) => Bind (StateT g m)

stateT :: Representable g => (Key g -> m (a, Key g)) -> StateT g m a

runStateT :: Indexable g => StateT g m a -> Key g -> m (a, Key g)

evalStateT :: (Indexable g, Monad m) => StateT g m a -> Key g -> m a

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

• evalStateT m s = liftM fst (runStateT m s)

execStateT :: (Indexable g, Monad m) => StateT g m a -> Key g -> m (Key g)

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

• execStateT m s = liftM snd (runStateT m s)

mapStateT :: Functor g => (m (a, Key g) -> n (b, Key g)) -> StateT g m a -> StateT g n b

liftCallCC :: Representable g => ((((a, Key g) -> m (b, Key g)) -> m (a, Key g)) -> m (a, Key g)) -> ((a -> StateT g m b) -> StateT g m a) -> StateT g m a

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

liftCallCC' :: Representable g => ((((a, Key g) -> m (b, Key g)) -> m (a, Key g)) -> m (a, Key g)) -> ((a -> StateT g m b) -> StateT g m a) -> StateT g m a

In-situ lifting of a callCC operation to the new monad. This version uses the current state on entering the continuation. It does not satisfy the laws of a monad transformer.

get :: MonadState s m => m s

Return the state from the internals of the monad.

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

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

put :: MonadState s m => s -> m ()

Replace the state inside the monad.

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.