An attempt at understanding and implementing a naive model for algebraic effects in JS.

naive-algebraic-effect.js

const Effect = (type, val) => ({ type, val });

const run = gen => {
  let r;
  while (!(r = gen.next()).done) {}
  return r.value;
};

function* handle(fn, map = {}) {
  const gen = fn();

  let r;
  let matchVal = undefined;

  while (!(r = gen.next(matchVal)).done) {
    if (r.value) {
      const matchFn = map[r.value.type];
      if (matchFn) {
        matchVal = matchFn(r.value.val);
      } else {
        yield r.value;
      }
    }
  }

  return r.value;
}

// Pseudo state
let state = 1;

// The "app";
const runner = handle(
  function*() {
    const number = yield* handle(
      function*() {
        const current = yield Effect("State", 1);
        yield Effect("Print", "Hello, world!");
        return current;
      },
      // "Pattern matching"
      {
        State: val => {
          state += val;
          return state;
        }
      }
    );
    return number + 1;
  },
  {
    Print: val => console.log(`Print: ${val}`)
  }
);

const output = run(runner);
console.log(output);

/*
OUTPUT
-> Print: Hello, world!
-> 3
*/