> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/perl6-rosettacode/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/perl6-rosettacode/programming_tasks/c/constrained_genericity.md).

# Constrained genericity

```perl
subset Eatable of Any where { .^can('eat') };

class Cake { method eat() {...} }

role FoodBox[Eatable] {
    has %.foodbox;
}

class Yummy does FoodBox[Cake] { }      # composes correctly
# class Yucky does FoodBox[Int] { }     # fails to compose

my Yummy $foodbox .= new;
say $foodbox;
```

```
Yummy.new(foodbox => {})
```