Add instance command registry and composition

app.go

@@ -0,0 +1,41 @@
+package commandkit
+
+import "fmt"
+
+// App owns one command registry and its application identity.
+type App struct {
+	identity Identity
+	commands map[string]*commandNode
+}
+
+type commandNode struct {
+	command  Command
+	reserved bool
+}
+
+// New creates an application with the supplied identity and initial commands.
+// The identity name must not be empty. The root help and version names are
+// reserved for generated commands.
+func New(identity Identity, commands ...Command) (*App, error) {
+	if identity.Name == "" {
+		return nil, fmt.Errorf("application identity name must not be empty")
+	}
+
+	app := &App{
+		identity: identity,
+		commands: map[string]*commandNode{
+			"help": {
+				command:  Command{name: "help", summary: "Show help for commands."},
+				reserved: true,
+			},
+			"version": {
+				command:  Command{name: "version", summary: "Show application version."},
+				reserved: true,
+			},
+		},
+	}
+	if err := app.Register(commands...); err != nil {
+		return nil, err
+	}
+	return app, nil
+}