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

["Huafeng Wang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-659?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15986392#comment-15986392 ] 

Huafeng Wang commented on BEAM-659:
-----------------------------------

Hi [~kenn], looks like it's a starter issue, can I assign it to myself?

> 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
>              Labels: easy, easyfix, starter
>
> {{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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