function RequestUserContext() {} // Used to signal request for user

// Understands the request for the user context
function runner(g, user) {
  const generator = g()

  let result = generator.next()

  do {
    if (result.value && result.value.constructor == RequestUserContext) {
      result = generator.next(user)

    } else {
      result = generator.next()
    }
  } while (!result.done)
}

// Usage somewhere down the road
runner(function * () {
  const user = yield new RequestUserContext()

  console.log('The current user: ', user)
}, { identity: 'Joe' })