Github user twalthr commented on the issue:
https://github.com/apache/flink/pull/2265
No, there is no FLIP about it. I think a discussion in JIRA or in this PR
should be enough. That's why I haven't documented it yet. I was inspired by
your
[document](https://docs.google.com/document/d/1KMUzvBAWSyQ39T8MyxUi0zNHyvLUnyGMPA7_RLSDpFw/edit).
You are right, `ScalarFunction` has many internal functions but they are not
exposed to the user, only 2 methods can be overriden.
An interface is not enough as it might be sometimes necessary to override
`getReturnType` and `getParameterType`.
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