[jira] [Created] (BEAM-659) WindowFn#isCompatible should provide a meaningful reason

["Kenneth Knowles (JIRA)" <ji...@apache.org>]

Kenneth Knowles created BEAM-659:
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             Summary: WindowFn#isCompatible should provide a meaningful reason
                 Key: BEAM-659
                 URL: https://issues.apache.org/jira/browse/BEAM-659
             Project: Beam
          Issue Type: Bug
          Components: sdk-java-core
            Reporter: Kenneth Knowles
            Priority: Minor


{{WindowFn#isCompatible(...)}} returns a true/false answer without any explanation of why; it suffers from a form of [boolean blindness|https://existentialtype.wordpress.com/2011/03/15/boolean-blindness/].

We alleviated this for {{Coder#isDeterministic()}} by switching to {{Coder#verifyDeterministic()}} which can raise an exception describing why it is not deterministic (it could just as easily have been a return value).

We should do the same for {{WindowFn#isCompatible(...)}}.

See here: http://stackoverflow.com/questions/39617897/inputs-to-flatten-had-incompatible-window-windowfns-when-cogroupbykey-with-calen

(a decent {{toString}} would go a long way, for immediate and trivial improvement)



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