capturing side effects in a task

task.js

const P = f => {
  const LazyRight = h => ({
    map: f => LazyRight(() => f(h())),
    then: (f, g) => g(h()),
    toString: () => `Resolve(${h})`,
  })
  const LazyLeft = h => ({
    map: f => LazyLeft(h),
    then: (f, g) => f(h()),
    toString: () => `Reject(${h})`,
  })
  const dust = Symbol()
  let L = dust
  let R
  f(rej => L = rej, res => R = res)
  return (L === dust) ? LazyRight(() => R) : LazyLeft(() => L)
};

P((rej, res) => {
  if (Math.random() > 0.5) {
    res(0)
  } else {
    rej(0)
  }
}).map(n => console.log(`log ${n}`) || n + 1)
  .map(n => console.log(`log ${n}`) || n + 1)
  .map(n => console.log(`log ${n}`) || n + 1)
  .map(n => console.log(`log ${n}`) || n + 1)
  .map(n => console.log(`log ${n}`) || n + 1)
  .then(
    n => console.log(`Reject - n: ${n}`),
    n => console.log(`Resolve - n: ${n}`)
  )