State monad in JS

state.js

// Functor
export const map = f => s => x0 => {
  const [v, x1] = s(x0);
  return [f(v), x1];
};

// Applicative
export const pure = a => x => [a, x];
export const ap = stf => stv => x0 => {
  const [f, x1] = stf(x0);
  const [v, x2] = stv(x1);
  return [f(v), x2];
};

// Monad
export const chain = f => s => x0 => {
  const [v, x1] = s(x0);
  return f(v)(x1);
};

state.test.js

import test from "ava";
import * as State from "../state";

test("monad", t => {
  // Flip flop between incrementing and decrementing
  const transition = State.chain(v => x => [v + x, x * -1]);
  const start = State.pure(0)
  t.snapshot(1 |> (start |> transition));
  t.snapshot(1 |> (start |> transition |> transition));
});

state.test.js.md

Generated by AVA.

monad

Snapshot 1

[
  1,
  -1,
]

Snapshot 2

[
  0,
  1,
]