Data commit

Task/Constrained-genericity/Wren/constrained-genericity.wren

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+// abstract class
+class Eatable {
+    eat() { /* override in child class */ }
+}
+
+class FoodBox {
+    construct new(contents) {
+        if (contents.any { |e| !(e is Eatable) }) {
+            Fiber.abort("All FoodBox elements must be eatable.")
+        }
+        _contents = contents
+    }
+
+    contents { _contents }
+}
+
+// Inherits from Eatable and overrides eat() method.
+class Pie is Eatable {
+    construct new(filling) { _filling = filling }
+
+    eat() { System.print("%(_filling) pie, yum!") }
+}
+
+// Not an Eatable.
+class Bicycle {
+    construct new() {}
+}
+
+var items = [Pie.new("Apple"),  Pie.new("Gooseberry")]
+var fb = FoodBox.new(items)
+fb.contents.each { |item| item.eat() }
+System.print()
+items.add(Bicycle.new())
+fb = FoodBox.new(items) // throws an error because Bicycle not eatable