State.md

State

The State datatype model values which can interact with a given state, both reading and writing to it. It allows treating stateful computations in a completely pure fashion.

Constructors

The State data structure has a single constructor which requires a function from the state to a pair consisting of the value and an updated version of the state.

/**
 * @template S
 * @template A
 */
final class State
{
    /**
     * @template T
     * @template B
     * @param callable(T): Pair<T, B> $runState
     * @return State<T, B>
     */
    public static function state(callable $runState): self
}

Its simplified type is

state :: (T -> Pair<T, B>) -> State<T, B>

Eliminators

The is only one eliminator for State, which just evaluates the inner action for a given state, returning a value and a possibly updated version of the state.

    /**
     * @param S $state
     * @return Pair<S, A>
     */
    public function runState($state): Pair

Its simplified type is

runState :: (State<S, A>, S) -> Pair<S, A>

Interpretation as an effect

The State data structure is used to model stateful computations in a pure fashion. A value of type State<S, A> represents a stateful computation interacting with a state of type S and returning a value of type A.

Typeclass instances

• StateApplicative
• StateApply
• StateFunctor
• StateMonad