add: type switch

interfaces/08-composition/discount.go

@@ -0,0 +1,17 @@
+package main
+
+// usually: include the discount method in the list.go
+// it is here for clarity
+
+// TODO: NEW
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
+	}
+
+	for _, it := range l {
+		if it, ok := it.(discounter); ok {
+			it.discount(ratio)
+		}
+	}
+}