How to forward errors using eventChannel?

Using generator functions I know how to create a way to use exceptions by throwing errors:

function * foo () {
  yield 1
  yield 2
  throw new Error('oops!')
  yield 3
}

try {
  for (const n of foo()) {
    console.log(n)
  }
} catch (err) {
  console.log(`caught '${err.message}'`)
}
console.log('done!')

outputs:

1
2
caught 'oops!'

as expected.

However if I have an eventChannel, I do not see how I can easily throw errors from it:

const myChannel = function () {
  return eventChannel(function (emitter) {
     const s = someSource()
        .on('thing', emitter)
        .on('error', () => console.log('I do not know what to do here')
        .on('done', () => emitter(END))
     return () => s.cancel()
  }
}

Is there a way I can easily propagate errors from the eventChannel into my sagas?

[yelouafi]

Actually there is a [kind of] support for errors in take effects: if a take is resolved with an Error it will throw in the generator. For example

const myChannel = function () {
  return eventChannel(function (emitter) {
     const s = someSource()
        .on('thing', emitter)
        .on('error', () => emitter(new Error(...) ) /* put an Error instance */ 
        .on('done', () => emitter(END))
     return () => s.cancel()
  }
}

function* saga() {
  const chan = yield call(myChannel)
  while(true) {
    try {
      const thing = yield take(chan)
    } catch(err) {
      // emitting an Error will throw here
    }
  }
}

Note that emitting an error will not abort the channel by default, if you want to end the channel after an error you’ll have to close it explicitly (for example in the catch block of the Saga)

[shinima]

@adipascu Added to the todo list in #1478