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https://issues.apache.org/jira/browse/CALCITE-7288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18037841#comment-18037841
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Dmitry Sysolyatin commented on CALCITE-7288:
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??was to leave the shuttle as is because so many do stream projects use it??
This is the main point. If we add a new method, it could potentially break
downstream projects, and it might do so silently. If we say “leave the shuttle
as is,” it means we should stop adding any methods to the {{RelShuttle}}
interface and just probably deprecate it.
??Maybe you should just implement your own base visitor in your own project??
It is reality, I assume most projects implement their own visitor to stay
stable in production. Our project is no exception, my last comment was about
pretty old story. But should it really be like this?
> Redesign RelShuttle
> -------------------
>
> Key: CALCITE-7288
> URL: https://issues.apache.org/jira/browse/CALCITE-7288
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Dmitry Sysolyatin
> Priority: Major
>
> This task is mainly for discussion and comes from [1]. We have an interface
> called {{RelShuttle}} that tries to do two things at once. It tries to follow
> the proper visitor pattern, but at the same time it also has a common method:
> {code:java}
> RelNode visit(RelNode other);{code}
> It would be better to remove this method from the interface and correctly use
> the visitor pattern (as for example RexVisitor does). That means, if we have
> a class {{A}} that extends {{{}RelNode{}}}, then {{RelShuttle}} should have a
> method like:
> {code:java}
> RelShuttle.visit(A rel) {code}
> Main concern:
> When we add a new method to `RelShuttle` for example
> {{visit(LogicalSnapshot)}}, all LogicalSnapshot related code will start
> calling {{RelShuttle.visit(LogicalSnapshot)}} instead of
> {{RelShuttle.visit(RelNode other)}}. But users might already handle snapshots
> in their {{RelShuttle.visit(RelNode other)}} method (as it happens with
> {{HintCollector}}/{{RelHomogeneousShuttle}}). It can silently break their
> code. Even if they have tests, they might think everything is fine while some
> issues still go unnoticed.
> With a proper visitor pattern, this would result in a compile-time error
> rather than silent behavior that could take a long time to debug
> Another option is to have only a single {{visit(RelNode other)}} method (like
> {{RelHomogeneousShuttle}} does, though in a specific way) and use pattern
> matching inside it.
> *But both approaches should not be used together in the same interface.*
> [1] [https://github.com/apache/calcite/pull/4620]
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