docs(stepfunctions): use JsonPath.DISCARD in place of null for input

[paulhcsun]

Update docs for use of sfn.JsonPath.DISCARD in place of null in TaskInput.fromObjects field value for languages that do not support null like Python.

Doc: https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_stepfunctions.TaskInput.html#:~:text=to%20a%20task.-,static%20fromObject(obj),-public%20static%20fromObject

Closes #16253.

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[mrgrain]

Does JsonPath.DISCARD not work?

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[paulhcsun]

@mrgrain JsonPath.DISCARD does work, I'm just not sure if the name of it is accurate for the purpose of replacing a null though, especially because the effect of using a null is not consistent across different cases. Taking one example provided in the issue:

"ItemResult": {
	"LastEvaluatedKey": JsonPath.DISCARD
     }

The name DISCARD here isn't the clearest of what we're trying to do but using NULL_VALUE is.

Actually reading the doc on it, it could make sense (Special string value to discard state input, output or result.). Perhaps I'm just thinking of DISCARD in a different context. Let me know what you think though.

[GavinZZ]

I didn't read the original issue, but just curious that does the following not work? The reason that it's completely ignoring null value is because on line 162, we explicitly skipped null or undefined scenario. If we don't want to ignore it, can't we just assign a value instead?

if (value === null || value === undefined) {
     Object.assign(ret, handlers.handleNull(key, null));
}

[paulhcsun]

I think users want to specifically be able to distinguish null from undefined in typescript, but at the same time the two are the same in other languages like Python (None) so a magic value of some sort will satisfy both cases.

[mrgrain]

@paulhcsun's right. This isn't an issue in TS (at least not as far as I know), but in jsii (languages) which cannot distinguish between null and undefined. But ASL (as a JSON-ish schema) sometimes requires users to use null

[mrgrain]

Actually reading the doc on it, it could make sense (Special string value to discard state input, output or result.). Perhaps I'm just thinking of DISCARD in a different context. Let me know what you think though.

I believe this has been named based on this feature and assuming that the only time someone actually wants null in the State Machine definition is for this reason. It's still the only reference to null that I can find in the Developer Guide, but that doesn't mean much.

https://docs.aws.amazon.com/step-functions/latest/dg/input-output-resultpath.html

If you look at the PRs and issue involved in JsonPath.DISCARD you might get some more context on it. Look out for comments and PRs by @RomainMuller

@mrgrain JsonPath.DISCARD does work, I'm just not sure if the name of it is accurate for the purpose of replacing a null though, especially because the effect of using a null is not consistent across different cases. Taking one example provided in the issue:

If we find that the naming/documentation for JsonPath.DISCARD isn't working for our users, we can definitely think about improving this. Aside from sprucing up the docs, I could imagine adding an alias for JsonPath.DISCARD, e.g. JsonPath.NULL.

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FWIW, after confirming JsonPath.DISCARD works in Python, the issue should be closed. But we can still discuss DX improvements.

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