doc-src/IsarAdvanced/Codegen/Thy/examples/class.ocaml

metis_tac: proper context (ProofContext.init it *not* sufficient);
simplified method setup;

module Nat = 
struct

type nat = Zero_nat | Suc of nat;;

end;; (*struct Nat*)

module Codegen = 
struct

type 'a null = {null : 'a};;
let null _A = _A.null;;

let rec head _B = function x :: xs -> x
                  | [] -> null _B;;

let rec null_option = None;;

let null_optiona () = ({null = null_option} : ('b option) null);;

let rec dummy
  = head (null_optiona ()) [Some (Nat.Suc Nat.Zero_nat); None];;

end;; (*struct Codegen*)