-- Abstract class.
class eatable
    function eat() end -- override in child class
end

class foodbox extends eatable
    private contents

    function __construct(contents)
        if !contents:checkall(|c| -> c instanceof eatable) then
            error("All foodbox elements must be eatable.")
        end
        self.contents = contents
    end

    function getContents() return self.contents end
end

-- Inherits from eatable and overrides eat() method.
class pie extends eatable
    function __construct(private filling) end

    function eat()
        print($"{self.filling} pie, yum!")
    end
end

-- Not an eatable
class bicycle end

local items = {new pie("Apple"), new pie("Gooseberry")}
local fb = new foodbox(items)
fb:getContents():foreach(|item| -> item:eat())
print()
items:insert(new bicycle())
fb = new foodbox(items)  -- throws an error because bicycle not eatable