Problem Statement

Constrained genericity or bounded quantification means that a parametrized type or function (see parametric polymorphism) can only be instantiated on types fulfilling some conditions, even if those conditions are not used in that function. Say a type is called "eatable" if you can call the function eat on it. Write a generic type FoodBox which contains a collection of objects of a type given as parameter, but can only be instantiated on eatable types. The FoodBox shall not use the function eat in any way (i.e. without the explicit restriction, it could be instantiated on any type). The specification of a type being eatable should be as generic as possible in your language (i.e. the restrictions on the implementation of eatable types should be as minimal as possible). Also explain the restrictions, if any, on the implementation of eatable types, and show at least one example of an eatable type.

Let's start with the solution:

module cg
    implicit none
    
    type, abstract :: eatable
    end type eatable
    
    type, extends(eatable) :: carrot_t
    end type carrot_t
    
    type :: brick_t; end type brick_t
    
    type :: foodbox
	class(eatable), allocatable :: food
    contains
        procedure, public :: add_item => add_item_fb
    end type foodbox
    
contains

    subroutine add_item_fb(this, f)
        class(foodbox), intent(inout) :: this
        class(eatable), intent(in)    :: f
        allocate(this%food, source=f)
    end subroutine add_item_fb
end module cg


program con_gen
    use cg
    implicit none
    
    type(carrot_t) :: carrot
    type(brick_t)  :: brick
    type(foodbox)  :: fbox
    
    ! Put a carrot into the foodbox
    call fbox%add_item(carrot)
    
    ! Try to put a brick in - results in a compiler error
    call fbox%add_item(brick)
    
end program con_gen