java rules should be able to depend on Skylark rules

[johnynek]

See:

https://github.com/bazelbuild/rules_scala/blob/master/test/BUILD#L15

The java libraries should probe to see if there are jars, transitive_runtime deps, etc...

[philwo]

Assigning to @laurentlb to get an idea how feasible this is.

[johnynek]

I would say there could be some contract that the java rule expects, and if skylark rules for scala, clojure, etc... follow that, the java rule would accept them.

This kind of jvm interop is pretty important for these other jvm languages.

[johnynek]

here is a possible work around approach:

bazel-contrib/rules_scala#18

[davidzchen]

@johnynek - Just out of curiosity, why do the deps labels for some of the scala targets not have the leading : in https://github.com/bazelbuild/rules_scala/blob/master/test/BUILD#L15?

scala_library(
    name = "HelloLib",
    srcs = ["HelloLib.scala"],
    deps = [
        "OtherJavaLib",
        "OtherLib",
        "MacroTest"
    ],
)

[johnynek]

@davidzchen it worked and I started removing them (fewer characters the better). Should targets always have : in the same package? Why does this work if so?

[ulfjack]

We've generally been using ":" to indicate rules, and "" to
indicate files.

Bazel handles both kinds of references as label references relative to the
current package. It performs lookup by first checking for a rule or an
output file, and if neither exists, it assumes that you're referencing a
source file. At the point in time where that happens, it does not check if
the source file exists.

On Thu, Mar 3, 2016 at 10:29 AM, P. Oscar Boykin notifications@github.com
wrote:

@davidzchen https://github.com/davidzchen it worked and I started
removing them (fewer characters the better). Should targets always have :
in the same package? Why does this work if so?

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#970 (comment).

[davidzchen]

Thanks for the explanations!

I think using : to distinguish between rules and files makes it more immediately clear whether a label referenced is a rule or file and would save time when reading BUILD files and understanding the dependencies between targets. There are cases where you could have input files that have no file extensions, in which case it would look like a rule unless you use the leading :. It would be better to continue using the same convention of using : to distinguish between rules and files for consistency.

@ulfjack - Is there a particular reason why Bazel allows referencing rules without the leading :? Would it be a good idea for us to enforce this convention?

[johnynek]

+1 to stronger checks. Would be a nice bazelrc option to require that.

[ulfjack]

Yes, I think it would be possible to have a stricter check.

On Fri, Mar 4, 2016 at 12:21 AM, P. Oscar Boykin notifications@github.com
wrote:

+1 to stronger checks. Would be a nice bazelrc option to require that.

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#970 (comment).

[jcoveney]

This conversation got derailed a bit around the :label, but I am curious how we want to deal with interop between JVM languages

I agree with @johnynek that the best way would be to create a contract that jar producing things can follow, and then some tools to pick them up and build a classpath