Cell level error handling

Hello,

When the evaluation of a cell throws an exception, the variable that was to be assigned the value becomes unavailable to subsequent calls, and all cells that reference that variables are now marked as error.

This is a problem when you have cases like this:

a = ... something that throws

b = ... something that doesn't throw

c = testThatsFalse ? a : b

It'd be better that c works.

I suggest that if the evaluation of a fails, a gets assigned an 'Error' type value, which can then be further processed. As in c = isError(a) ? b : a.

Happy to work on a PR if you think this could be a good idea.
This obviously would impact the way the minimap works, but I don't think there's a case where old notebooks would suffer from this change.
It's a one-way change though...

[mbostock]

Adding a promiseof operator would be one way to allow error handling (as well as parallelism).

Just throwing my thoughts out here.

Having the users handle the promise is a little against what Observable usually does (taking care of promises). And it's going to be cumbersome as in 90% of cases, users will want to wrap the call that may fail in a try-await-promiseof-catch block that's unfortunately quite verbose.

Why not treat every variable references as promises to be awaited? ie. Making promiseof the general case rather than the special case? There may be implications around async generators I am not grasping though.

[tomlarkworthy]

Do you think we could consume events to avoid dataflow chaining (yet causing a cell to be observed still)

neverEmitEventsCell = {
    while (true) {
        await promiseof eventEmitterCell
    }
}