import { todo } from './todo.js'
/** @import { State } from './types' */

/**
 * Optional (Exercise 4.7). A `State<S, A>` is a plain function from a state to a pair: `s => [value, nextS]`.
 * It is `Random<A>` with the seed replaced by any state type `S`. Nothing in the game uses this file.
 */

/** A State that gives `a` and leaves the state untouched: `s => [a, s]`.
 * @type {<S, A>(a: A) => State<S, A>} */
export const pure = (a) => todo('m04')

/** 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) => todo('m04')

/** Run `st`, feed its value to `f` to pick the next State, 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) => todo('m04')

/** A State whose value is the current state, which it leaves untouched: `s => [s, s]`. (A value, not a function call.)
 * @type {<S>(s: S) => [S, S]} */
export const get = (s) => todo('m04')

/** A State that replaces the state with `s` and gives `undefined`: whatever the old state, `[undefined, s]`.
 * @type {<S>(s: S) => State<S, undefined>} */
export const put = (s) => todo('m04')

/** 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) => todo('m04')

/** 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) => todo('m04')