Support mill init from an existing Maven project (1500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 1500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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We should be able to run ./mill init to do a best effort conversion of an existing Maven project to Mill:

• Code-generate a build.mill containing a MavenModulefor the root pom.xml, the package.mill for any subfolder pom.xmls
• Convert the main and test dependencies on third-party libraries to ivyDeps
• Convert inter-subproject dependencies to moduleDeps
• Set any necessary javacOptions
• Implement PublishModule with relevant metadata
• Support POM inheritance, dependency management, etc.

Such a conversion will never be 100%, but if we can automate 80% of the conversion that will already be a huge help for people migrating to Mill or even just trying it out

Some prior art that can be used as reference:

• Gradle's Maven Importer https://docs.gradle.org/current/userguide/build_init_plugin.html#sec:pom_maven_conversion
• SBT's POM Reader Plugin https://github.com/sbt/sbt-pom-reader

[arturaz]

Is there any infrastructure for this already? For example, does mill init entry point exists?

[lihaoyi]

@arturaz mill init already exists, and uses giter8 to initialize a repo based on some github URL:

mill/main/src/mill/main/MainModule.scala

Line 403 in 7732b0f

def init(evaluator: Evaluator, args: String*): Command[Unit] = Target.command {

It should be pretty easy to add logic to check if there's an existing pom.xml in the project root and initialize based on that instead

[arturaz]

I'll try to implement this :)

[lihaoyi]

@arturaz go for it!