Re: Create a constraint between variant type and data list

[oleg@okmij.org]

Sylvain Le Gall wrote:

> I would like to define:
>
> type license = GPL | LGPL
>
> and
>
> let data = [ GPL, "GNU Public license";
>              LGPL, "GNU Lesser General Public license" ]
>
> I would like to enforce that all variants of license are in the
> association list.

In other words, you would like a set of values of a distinct type,
each of which is associated with a printable string. One should be
able to add to the set in some `authorized way'; one should not be
able to make a value of that distinct type in an unauthorized way
(that is, by accident). Right?

It seems the following solution then would satisfy the specification:

module LICENSE : sig
  type t
  val print_lic : t -> string
  val gpl  : t
  val lgpl : t 
  (* more can be added *)
end = struct
  type t = string
  let print_lic x = x
  let gpl  = "GNU Public license"
  let lgpl = "GNU Lesser General Public license"
end;;

(* Usage example *)
type package = {pkg_name : string; pkg_lic : LICENSE.t};;

let packages = [{pkg_name = "gcc";    pkg_lic = LICENSE.gpl};
	        {pkg_name = "lgpled"; pkg_lic = LICENSE.lgpl}]
;;

let rec pr_lics = function [] -> ()
 | {pkg_lic = l}::t -> Printf.printf "%s\n" (LICENSE.print_lic l);
                       pr_lics t
;;

let rec all_of_lic l = function [] -> []
  | {pkg_name = name; pkg_lic = l'}::t when l' == l -> name :: all_of_lic l t
  | _::t -> all_of_lic l t
;;

all_of_lic LICENSE.lgpl packages;;


One can't overlook specifying the description string. One can't create
new values of the type LICENSE.t `by accident' (the only possible
values of the type are the ones exported by the module).