Control.Monad.State

type State s = StateT s Identity

The State monad is a synonym for the StateT monad transformer, applied to the Identity monad.

runState :: forall s a. State s a -> s -> Tuple a s

Run a computation in the State monad

evalState :: forall s a. State s a -> s -> a

Run a computation in the State monad, discarding the final state

execState :: forall s a. State s a -> s -> s

Run a computation in the State monad, discarding the result

mapState :: forall s a b. (Tuple a s -> Tuple b s) -> State s a -> State s b

Change the type of the result in a State action

withState :: forall s a. (s -> s) -> State s a -> State s a

Modify the state in a State action

class (Monad m) <= MonadState s m | m -> s where

The MonadState s type class represents those monads which support a single piece of mutable state of type s.

• state f updates the state using the function f.

An implementation is provided for StateT, and for other monad transformers defined in this library.

Laws:

• do { get ; get } = get
• do { put x ; put y } = put y
• do { put x ; get } = put x $> x
• do { s <- get ; put s } = pure unit

Members

• state :: forall a. (s -> (Tuple a s)) -> m a

put :: forall m s. MonadState s m => s -> m Unit

Set the state.

modify_ :: forall s m. MonadState s m => (s -> s) -> m Unit

modify :: forall s m. MonadState s m => (s -> s) -> m s

Modify the state by applying a function to the current state. The returned value is the new state value.

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

Get a value which depends on the current state.

get :: forall m s. MonadState s m => m s

Get the current state.

newtype StateT s m a

The state monad transformer.

This monad transformer extends the base monad with the operations get and put which can be used to model a single piece of mutable state.

The MonadState type class describes the operations supported by this monad.

Constructors

• StateT (s -> m (Tuple a s))

Instances

• Newtype (StateT s m a) _
• (Functor m) => Functor (StateT s m)
• (Monad m) => Apply (StateT s m)
• (Monad m) => Applicative (StateT s m)
• (Monad m, Alt m) => Alt (StateT s m)
• (Monad m, Plus m) => Plus (StateT s m)
• (Monad m, Alternative m) => Alternative (StateT s m)
• (Monad m) => Bind (StateT s m)
• (Monad m) => Monad (StateT s m)
• (MonadRec m) => MonadRec (StateT s m)
• (MonadZero m) => MonadZero (StateT s m)
• (MonadPlus m) => MonadPlus (StateT s m)
• MonadTrans (StateT s)
• Lazy (StateT s m a)
• (MonadEffect m) => MonadEffect (StateT s m)
• (MonadCont m) => MonadCont (StateT s m)
• (MonadThrow e m) => MonadThrow e (StateT s m)
• (MonadError e m) => MonadError e (StateT s m)
• (MonadAsk r m) => MonadAsk r (StateT s m)
• (MonadReader r m) => MonadReader r (StateT s m)
• (Monad m) => MonadState s (StateT s m)
• (MonadTell w m) => MonadTell w (StateT s m)
• (MonadWriter w m) => MonadWriter w (StateT s m)
• (Monad m, Semigroup a) => Semigroup (StateT s m a)
• (Monad m, Monoid a) => Monoid (StateT s m a)

class MonadTrans t  where

The MonadTrans type class represents monad transformers.

A monad transformer is a type constructor of kind (* -> *) -> * -> *, which takes a Monad as its first argument, and returns another Monad.

This allows us to add additional effects to an existing monad. By iterating this process, we create monad transformer stacks, which contain all of the effects required for a particular computation.

The laws state that lift is a Monad morphism.

Laws:

• lift (pure a) = pure a
• lift (do { x <- m ; y }) = do { x <- lift m ; lift y }

Members

• lift :: forall m a. Monad m => m a -> t m a

withStateT :: forall s m a. (s -> s) -> StateT s m a -> StateT s m a

Modify the final state in a StateT monad action.

runStateT :: forall s m a. StateT s m a -> s -> m (Tuple a s)

Run a computation in the StateT monad.

mapStateT :: forall s m1 m2 a b. (m1 (Tuple a s) -> m2 (Tuple b s)) -> StateT s m1 a -> StateT s m2 b

Change the result type in a StateT monad action.

execStateT :: forall s m a. Functor m => StateT s m a -> s -> m s

Run a computation in the StateT monad discarding the result.

evalStateT :: forall s m a. Functor m => StateT s m a -> s -> m a

Run a computation in the StateT monad, discarding the final state.