Flow: Issue with `${generator.next().value}`

[ghost]

Flow reports error:

[flow] [flow] undefined (This type cannot be coerced to string)
(property) IteratorResult.value: any
[Flow]
value: number | void | void

The code executes fine. Is there something wrong with it or we see a flow bug? (I installed latest flow etc. just yesterday.)

The same code for copying:
`// @flow

function * fibonacciSequenceMaker(first, second) {
let [previous, current] = [first, second];
yield previous; yield current;
while (true) {
[previous, current] = [current, previous + current];
yield current;
}
}

function * fibonacciSequenceEvensMaker(first, second, filter) {
for (let thisResult of fibonacciSequenceMaker(first, second))
if (filter(thisResult)) yield thisResult;
}

let result = '';
let generator = fibonacciSequenceEvensMaker(2, 7, (v) => v%2 === 0);
for (let i = 0; i < 20; i++) result += ${generator.next().value},;
console.log(result);
`

[rafales]

value may be actually undefined (When done is true). You either need to check if it's not undefined or check if done is false.

[ghost]

That makes the warning gone:

However, the generator is intended as infinite one. Can you give example how "done" could be true in the same code?

[rafales]

@karlis-repsons flow doesn't know that this is an infinite generator, therefore it thinks that value may be undefined.

[ghost]

Ok, so this would be asking too much: add infinite generators detection to Flow?

[rafales]

Don't know, but you can tell flow that you know for sure that it's not null:

import invariant from 'invariant';

// ...
const value = generator.next().value;
invariant(value != null, 'this should never be null');

or

const value = generator.next().value;
if (value == null) { throw new Error('this should never be null') }

[ghost]

I am too not sure if Flow should try to detect simple infinite generators. But since they could become complicated, thanks for the tip to use invariant.

[vkurchatkin]

Ok, so this would be asking too much: add infinite generators detection to Flow?

That's not just complicated, that's truly impossible: https://en.wikipedia.org/wiki/Halting_problem