Detached generators don’t resume when reattached

[mbostock]

For example, given this generator:

const j = (async function* () {
  for (let j = 0; true; ++j) {
    yield j;
    await new Promise((resolve) => setTimeout(resolve, 1000));
  }
})();

If you reference the value of j like so:

The value of j is: ${j}.

Then everything works as expected. But, if you delete the reference to j, save, and then restore the reference to j, then j will resolve to undefined instead of incrementing once per second. I would guess that this has something to do with the generator being consumed lazily, and something bad is happening if there is no longer a downstream observer. 🤔

[mbostock]

I spent a bit of time looking at this. It’s tricky. Some observations…

When a generator variable is invalidated, the generator is returned (terminated) here:

https://github.com/observablehq/runtime/blob/037fd2de7f1b461025d2cac91acd05639adce722/src/runtime.js#L269

When a variable becomes no longer reachable, the variable is invalidated here:

https://github.com/observablehq/runtime/blob/037fd2de7f1b461025d2cac91acd05639adce722/src/runtime.js#L108

The combination of this means that the generator is terminated when the associated variable is no longer reachable. But the problem is resumption: recomputing the variable j doesn’t create a new generator; it simply returns the existing one that was returned by cell 42. And since that generator has already been returned, the subsequent call to generator.next returns {done: true, value: undefined}.

I think probably this means that variables that are no longer reachable should not be invalidated — they’re still valid, just not reachable. But that means that a variable that becomes reachable doesn’t necessarily need to be computed. And it means that we don’t want to keep pulling from the generator while it’s not reachable; we should wait until it becomes reachable again. 🤔