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https://issues.apache.org/jira/browse/CALCITE-2458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16617961#comment-16617961
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Vladimir Sitnikov commented on CALCITE-2458:
--------------------------------------------
Julian, we are not discussing things like "we must rewrite everything to Kotlin
in three weeks".
Of course, we could decline contributions that exercise extreme corner cases of
the language.
Java could produce hard to read/understand/understand idioms, and we mitigate
those risks by reviewing the changes and refactoring the code.
Note: current codebase contains lots of languages (Java, XML, SQL, JUnit,
Markdown, HTML, CSS), so adding Kotlin is not like "changing half of the
codebase".
> Evaluate use of Kotlin for unit tests
> -------------------------------------
>
> Key: CALCITE-2458
> URL: https://issues.apache.org/jira/browse/CALCITE-2458
> Project: Calcite
> Issue Type: Improvement
> Affects Versions: 1.17.0
> Reporter: Vladimir Sitnikov
> Assignee: Julian Hyde
> Priority: Major
>
> It looks like Kotlin might simplify writing tests:
> 1) Calcite tests often create expressions (linq4j, rex, sql, etc), and the
> order of elements is "backwards".
> For instance, "x AND (y OR z)" becomes {{and(x, or(y, z))}} at best. Writing
> and updating such code is a bit tedious. It seems like {{AND}} and {{OR}}
> could be infix functions (see
> [https://kotlinlang.org/docs/reference/functions.html#infix-notation|https://kotlinlang.org/docs/reference/functions.html#infix-notation]
> )
> 2) [extension
> functions|https://kotlinlang.org/docs/reference/extensions.html#extensions]
> Calcite tests often tend to create DSLs for testing (e.g. CalciteAssert,
> Tester, and so on). The idea there is to enable fluent APIs and somehow tame
> the complexity. The problem there is Java is not that suitable for building
> DSLs.
> Extension methods in Kotlin allow to "add a method to existing class", and it
> might be helpful for cases like
> {{parser.parse("...").assertConvertsTo("...")}} where {{assertConvertsTo}} is
> an extension method (in Java it could be a static method in CalciteAssert
> class)
> 3) [data classes|https://kotlinlang.org/docs/reference/data-classes.html].
> Apparently, Calcite deals with data, and data classes could help here as well.
> 4) [default
> parameters|https://kotlinlang.org/docs/reference/functions.html#default-arguments]
> 5) [multiline string
> literals|https://kotlinlang.org/docs/reference/basic-types.html#string-literals]
> I think it would be much better to co-locate SqlToRel test code and its
> expected results, so one can see the test code and expectations.
> 6) Re Checkstyle: there's a standard code style for Kotlin (and it can be
> verified automatically), however I am not sure we could configure it in the
> way we have Checkstyle rules. Calcite uses parenthesis a lot, and I am not
> sure how Kotlin would deal with it.
> It looks like adding Kotlin as a {{<scope>test</scope>}} should not be a
> problem, so I wonder if that is feasible.
> PS. Using Kotlin for regular Calcite code is a different story, and I am not
> sure I want to open that discussion (well, I would love to, yet it might be a
> major change with ripples here and there). I just think it should be safer to
> try writing some TEST code for Calcite in Kotlin, then evaluate it for other
> cases (if necessary at all).
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