// m04 state (optional, Exercise 4.7): the general pattern behind Random. A State<S, A> is a function
// s => [A, nextS]: it reads a state, produces a value and hands on a new state. Random<A> is State<seed, A>.
// Nothing in the game uses this file; it is here to show that Random was one example of a bigger idea.
/** @import { State } from './types' */

/** A State that gives `a` and leaves the state untouched.
 * @type {<S, A>(a: A) => State<S, A>} */
export const pure = (a) => (s) => [a, s]

/** Transform the value a State produces; the state threading is unchanged.
 * @type {<S, A, B>(st: State<S, A>, f: (a: A) => B) => State<S, B>} */
export const map = (st, f) => (s) => {
  const [a, next] = st(s)
  return [f(a), next]
}

/** Run `st`, let `f` pick the next State from its value, and run that with the state `st` left behind.
 * @type {<S, A, B>(st: State<S, A>, f: (a: A) => State<S, B>) => State<S, B>} */
export const flatMap = (st, f) => (s) => {
  const [a, next] = st(s)
  return f(a)(next)
}

/** A State whose value is the current state, which it leaves untouched.
 * @type {<S>(s: S) => [S, S]} */
export const get = (s) => [s, s]

/** A State that replaces the state with `s` and gives `undefined` (there is no interesting value).
 * @type {<S>(s: S) => State<S, undefined>} */
export const put = (s) => () => [undefined, s]

/** A State that replaces the state `s` with `f(s)` and gives `undefined`.
 * @type {<S>(f: (s: S) => S) => State<S, undefined>} */
export const modify = (f) => (s) => [undefined, f(s)]

/** Run a State from an initial state: `[value, finalState]`.
 * @type {<S, A>(st: State<S, A>, s: S) => [A, S]} */
export const run = (st, s) => st(s)