Task monad with some examples at the bottom

task-monad.js

import { pipe } from './utils';

export const Task = (fork) => ({
  fork,
  map: (f) => Task((rej, res) => fork(rej, pipe(f, res))),
  chain: (f) => Task((rej, res) => fork(rej, (x) => f(x).fork(rej, res))),
  fold: (f, g) =>
    Task((rej, res) =>
      fork(
        (x) => f(x).fork(rej, res),
        (x) => g(x).fork(rej, res)
      )
    ),
});

Task.of = (x) => (rej, res) => res(x);
Task.fromPromised = (fn) => (...args) =>
  Task((rej, res) =>
    fn(...args)
      .then(res)
      .catch(rej)
  );
};
Task.rejected = (x) => Task((rej, res) => rej(x));




const lifted = Task.of('pure')
lifted(x => console.error(`\n### x: \n\t${x}`), x => console.log(`\n### x: \n\t${x}`))


const rejected = Task.rejected(2)
rejected.fork(x => console.error(`\n### x: \n\t${x}`), x => console.log(`\n### x: \n\t${x}`))

const setTimeoutTask = Task((rej, res) => {
  setTimeout(() => {
    // res('resolved set timeout task');
    rej('rejected set timeout task');
  }, 300);
})

setTimeoutTask.fork(x => console.error(`\n### x: \n\t${x}`), x => console.log(`\n### x: \n\t${x}`))

Thanks to @DrBoolean for all his contributions to the fp in js world.