Follow a Dependency

A project must declare every package whose modules it imports. Sometimes another dependency already selects the package version you want to use. Declare follows to share that selection without specifying another archive URL, content hash, or local path.

For example, suppose a fictional garden package depends on a seeds package. An application that imports modules from both can use:

dependencies = {
    "garden": (path="../garden"),
    "seeds": (follows="garden.seeds"),
}

The application can then write import seeds or import seeds.catalog. The seeds declaration points to the same selected project as garden's seeds dependency. The dependency key must still match the selected project's name.

The reference is a path through dependency names in Build.act. Its first component names a dependency of the declaring project, and each remaining component names a dependency of the project reached so far. Longer references such as toolbox.garden.seeds are supported, as are references whose target is itself a following dependency. The reference must contain at least two names, and every named dependency must exist. Cyclic references are errors.

follows does not choose a version or override another declaration. Acton first resolves the concrete path and archive dependencies, including root pins and project deduplication, then binds following dependencies to the selected projects. If another concrete declaration causes garden to use a different seeds version, the application's following dependency uses that same version. Adding the following declaration cannot change which version wins.

Relative paths still belong to the project that declares them. If garden declares seeds with path="../seeds", that path is resolved relative to garden's directory, regardless of where the application lives.

Use follows on its own in a dependency tuple. It cannot be combined with path, url, hash, repo_url, or repo_ref. To select a different version yourself, replace the following declaration with a concrete dependency.

acton pkg upgrade preserves following declarations. Upgrade the dependency that supplies the concrete version instead. A --dep seeds=... override changes concrete seeds declarations; following declarations observe that selection without becoming concrete dependencies themselves.