Golang Uber Dig

1. Keep the container at the composition root — never pass *dig.Container as a parameter; treat it like a plumbing detail of main(). Service-locator patterns defeat the testability gains of DI. 2. Depend on interfaces, not concrete types — lets you swap implementations in tests without touching production code, and lets you use dig.As to expose narrow interfaces from wide structs. 3. Prefer parameter objects (dig.In structs) once a constructor has 4+ dependencies — call sites stay readable and adding a new dependency is a one-line change instead of a signature break. 4. Group registration by module (one file per module that calls c.Provide for its types) — review and refactoring become a per-module concern, and you can extract a module into a fx.Module later without rewriting wiring. 5. Validate the graph eagerly in tests — call c.Invoke against the composition root in CI to surface missing providers at boot time, not at first request. DryRun(true) skips constructor execution. 6. Return errors from constructors instead of panicking — dig wraps them with the dependency path, which makes the failure point obvious.