cfgkit

type Config struct {
	Port     int           `env:"PORT"      default:"8080" doc:"HTTP listen port"`
	Timeout  time.Duration `env:"TIMEOUT"   default:"15s"`
	DBURL    string        `env:"DATABASE_URL,required"    doc:"Postgres connection string"`
	APIKey   string        `env:"API_KEY"   secret:"true"  doc:"Upstream API key"`
}

cfg, res, err := cfgkit.Load[Config](
	cfgkit.WithSources(
		cfgkit.FromFiles(".env", ".env.local"),
		cfgkit.FromEnviron(),   // last = highest precedence
	),
)

cfgkit.FromEnviron()
cfgkit.FromPrefixedEnviron("SVC_")
cfgkit.FromFiles(".env", ".env.local")     // missing file is not an error
cfgkit.FromFlagSet(fs)                     // stdlib flag
cfgkit.FromMap(m)                          // the test seam

secrets, _ := vault.ReadAll(ctx, "secret/data/app")     // pre-load: Lookup runs once per field
src := cfgkit.SourceFunc("vault", func(key string) (string, bool, error) {
	v, ok := secrets[key]
	return v, ok, nil
})

src := cfgkit.StructuredFunc("config.yaml", func(dst any) error {
	b, err := os.ReadFile("config.yaml")
	if err != nil {
		return err
	}
	return yaml.Unmarshal(b, dst)   // the yaml dependency is YOURS
})