rename Subsystem -> Module, add NewLifecycleL

module.go

@@ -0,0 +1,49 @@
+package app
+
+import (
+	"fmt"
+	"log/slog"
+)
+
+// Module represents a sub-system of the application, with its setup and
+// teardown functions and dependencies.
+type Module struct {
+	logger   *slog.Logger
+	name     string
+	setup    setupFn
+	teardown teardownFn
+	depends  []*Module
+}
+
+// ModuleOpts contains user-exposed options when defining a module.
+type ModuleOpts struct {
+	// Setup is the setup function for the module.
+	Setup setupFn
+	// Teardown is the teardown function for the module.
+	Teardown teardownFn
+	// Depends is a list of modules that this module depends on.
+	Depends []*Module
+}
+
+// NewModule creates a new Module instance with the given name and options.
+func NewModule(name string, opts ModuleOpts) *Module {
+	return &Module{
+		name:     name,
+		setup:    opts.Setup,
+		teardown: opts.Teardown,
+		depends:  opts.Depends,
+	}
+}
+
+// Logger returns the logger for the module.
+func (s *Module) Logger() *slog.Logger {
+	if s.logger == nil {
+		panic(fmt.Sprintf("subsytem %s used before logger was initialized", s.name))
+	}
+	return s.logger
+}
+
+// Name returns the name of the module.
+func (s *Module) Name() string {
+	return s.name
+}