Source code in the ats programming language

(* The Rosetta Code linear list type can contain any vt@ype.
   (The ‘@’ means it doesn’t have to be the size of a pointer.
   You can read {0 <= n} as ‘for all non-negative n’. *)
dataviewtype rclist_vt (vt : vt@ype+, n : int) =
| rclist_vt_nil (vt, 0)
| {0 <= n} rclist_vt_cons (vt, n + 1) of (vt, rclist_vt (vt, n))

(* A lemma one will need: lists never have negative lengths. *)
extern prfun {vt : vt@ype}
lemma_rclist_vt_param
          {n : int}
          (lst : !rclist_vt (vt, n)) : [0 <= n] void

(* Proof of the lemma. *)
primplement {vt}
lemma_rclist_vt_param lst =
  case+ lst of
  | rclist_vt_nil () => ()
  | rclist_vt_cons _ => ()