Testing JavaScript Promise monad laws

promise-monad.js

await (async () => {
  function wrap(p) {
    return p.then(
      (res) => ["ok", res],
      (err) => ["error", err]
    );
  }

  async function compare(title, pa, pb) {
    const a = await wrap(pa);
    const b = await wrap(pb);
    console.info(title, { a, b });
  }

  async function leftIdentity(f, x) {
    await compare("left identity", f(x), Promise.resolve(x).then(f));
  }

  async function rightIdentity(p) {
    await compare(
      "right identity",
      p,
      p.then((x) => Promise.resolve(x))
    );
  }

  async function associativity(p, f, g) {
    await compare(
      "associativity",
      p.then(f).then(g),
      p.then((x) => f(x).then(g))
    );
  }

  await leftIdentity((x) => Promise.resolve(x), "foo");
  await leftIdentity((x) => Promise.reject("foo err"), "foo");

  await rightIdentity(Promise.resolve("bar"));
  await rightIdentity(Promise.reject("bar err"));

  await associativity(
    Promise.resolve("baz"),
    (x) => Promise.resolve(x),
    (x) => Promise.resolve(x)
  );
  await associativity(
    Promise.reject("baz err 1"),
    (x) => Promise.resolve(x),
    (x) => Promise.resolve(x)
  );
  await associativity(
    Promise.resolve("baz"),
    (x) => Promise.reject("baz err 2"),
    (x) => Promise.resolve(x)
  );
  await associativity(
    Promise.resolve("baz"),
    (x) => {
      throw "baz throw 2";
    },
    (x) => Promise.resolve(x)
  );
  await associativity(
    Promise.resolve("baz"),
    (x) => Promise.resolve(x),
    (x) => Promise.reject("baz err 3")
  );
  await associativity(
    Promise.resolve("baz"),
    (x) => Promise.resolve(x),
    (x) => {
      throw "baz throw 3";
    }
  );
})();