promotions

pkg/promotions/eval.go

@@ -100,14 +100,33 @@ func NewContextFromCart(g *cart.CartGrain, opts ...ContextOption) *PromotionEval
 // PromotionService evaluates PromotionRules against a PromotionEvalContext.
 type PromotionService struct {
 	tracer Tracer
+	// effects is the per-action-type behaviour registry. Each Effect both applies
+	// its action to the cart (when the owning rule qualifies) and reports its own
+	// progress, so any effect type can surface "spend X more for ..." nudges —
+	// even one that has already applied (e.g. a tiered discount at 5% pointing at
+	// the next 9% tier). Register a new Effect to add a new action type.
+	effects map[ActionType]Effect
 }
 
-// NewPromotionService constructs a PromotionService with an optional tracer.
+// NewPromotionService constructs a PromotionService with an optional tracer and
+// the built-in effect handlers.
 func NewPromotionService(t Tracer) *PromotionService {
 	if t == nil {
 		t = NoopTracer{}
 	}
-	return &PromotionService{tracer: t}
+	return &PromotionService{
+		tracer:  t,
+		effects: defaultEffects(),
+	}
+}
+
+// RegisterEffect adds (or overrides) the handler for an action type, so new
+// applied/pending effects can be plugged in without touching the engine.
+func (s *PromotionService) RegisterEffect(e Effect) {
+	if s.effects == nil {
+		s.effects = map[ActionType]Effect{}
+	}
+	s.effects[e.Type()] = e
 }
 
 // EvaluationResult holds the outcome of evaluating a single rule.