type term = Var of int | Lambda of term | App of term * term
          | Constant of int | Primitive of primitive * term list
and value = Int of int | Closure of term * environment
and environment = value list
and primitive = value list -> value
let rec eval env term =
  match term with
    Var n -> List.nth env n
  | Lambda a -> Closure(a, env)
  | App(a, b) ->
      let (Closure(c, envp)) = eval env a in
      let v = eval env b in
      eval (v :: envp) c
  | Constant n ->
      Int n
  | Primitive(p, arguments) ->
      p (List.map (eval env) arguments)