Source Plugins

[lefou]

Since Mill got support for meta-builds, it should be rather easy to support source-plugins. What are source plugins? Mill plgins, that aren't consumed as binary artifact but instead are distributes as source code and will be build and loaded by the local Mill process.

In the meta build, instead of adding a Mill plugin as binary dependency to mvnDeps we want to add it to moduleDeps, where each plugin is represented by a sub-module.

I can imagine different kind of plugin sub-module, but it roughly boils down to:

• fetch the plugin from some remote source or version control system
• build it against the current Mill version

This of course leaves out some details. Here are some more thoughts:

• plugins could provide a package.mill or a plugin_package.mill, so Mill can reuse that.
• it should be possibly to override anything from build description to code
• we probably want to identify major building blocks and make the separate usable components, e.g.
  • checkout and update remote source trees
  • patch a remote source tree

Once, we have a more solid idea and implementation, we can think of a generic machanism as easy as adding a source plugin to the YAML frontmatter.

//| sourcePlugins:
//| - git:...

Why do we want source plugin?

• No need to respect binary compatibility, only source compatibility is a concern
• No need to setup a publish and distribution process, e.g. create a Sonatype account
• Plugins are most likely immediately available to unstable Mill version
• Testing and contributing to plugins is easier

Downsides?

• Potentially less future-proof build chain, since the plugin sources might disappear (we might want to develop some snapshot feature where we can freeze and semi-vendor a plugin in the build tree)