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 type Eatable = sig
  type t
  val eat : t -> unit
end


module MakeFoodBox(A : Eatable) = struct
  type elt = A.t
  type t = F of elt list
  let make_box_from_list xs = F xs
end


type banana = Foo (* a dummy type *)

module Banana : Eatable with type t = banana = struct
  type t = banana
  let eat _ = print_endline "I'm eating a banana"
end


module EatFloat : Eatable with type t = float = struct
  type t = float
  let eat f = Printf.printf "I'm eating %f\n%!" f
end


module BananaBox = MakeFoodBox (Banana)
module FloatBox = MakeFoodBox (EatFloat)

let my_box = BananaBox.make_box_from_list [Foo]
let your_box = FloatBox.make_box_from_list [2.3; 4.5]