On Fri, 5 Jan 2007, Nathaniel Gray wrote:
> On 12/29/06, Mattias Engdegård <mattias@virtutech.se> wrote:
> > Is there a reason for this? To my innocent eyes, code like
> >
> >   type length = Length of int
> >
> > looks quite reasonable and could be useful at times.
>
> I agree.  Sadly, the ocaml devs don't.
>
> http://caml.inria.fr/mantis/view.php?id=3978
>
> As Xavier points out, one can use modules to hide basic types, but
> this is pretty clumsy in practice.  There's rumored to be a solution
> using phantom types, but my attempts don't work:

You need to use modules to make phantom types work. Something like this

module type BOINK =
  sig
    type 'a t
    val inj : int -> 'a t
    val prj : 'a t -> int
    val plus : 'a t -> 'a t -> 'a t
  end;;

module Boink : BOINK =
  struct
    type 'a t = int
    let inj n = n
    let prj t = t
    let plus x y = x + y
  end;;

let f : string Boink.t = inj 20;;
let g : int Boink.t = inj 30;;

Boink.plus f g;;

I didn't compile that, but you get the idea...

-- Brian