[Caml-list] phantom type

[Jiten Pathy <jpathy@fssrv.net>]

Hello,
Trying to use phantom types instead of gadt for well-constructed term
example. Is it possible to define an evaluator eval : 'a term -> 'a
using phantom types? Something like following doesn't work.

type expr = Zero | Succ of expr | Iszero of expr;;
type 'a term = Expr of expr;;

(* ways to construct a term *)
let zero : int term = Expr Zero;;
let succ : int term -> int term = fun (Expr x) -> Expr (Succ x);;
let iszero : int term -> bool term = fun (Expr x) -> Expr (Iszero x);;

let rec eval : 'a.'a term -> 'a = fun (type a) (t : a term) -> match t with
| Expr Zero -> 0
| Expr (Succ e) -> eval (Expr e) + 1
| Expr (Iszero e) -> if (eval (Expr e) == 0) then true else false
;;