> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/sidef-lang/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/sidef-lang/programming_tasks/c/constrained_genericity.md).

# Constrained genericity

```ruby
class FoodBox(*food { .all { .respond_to(:eat) } }) { }
 
class Fruit { method eat { ... } }
class Apple < Fruit {  }
 
say FoodBox(Fruit(), Apple()).dump  #=> FoodBox(food: [Fruit(), Apple()])
say FoodBox(Apple(), "foo")         #!> ERROR: class `FoodBox` !~ (Apple, String)
```