(Re)add recursion support

eval.ml

@@ -285,7 +285,14 @@ let rec eval env ast =
           begin match args with
             | [] -> f
             | a::args ->
-              let env = Env.bind (var, aux a) env in
+              let itself = match v with Var v -> Some v | _ -> None in
+              let env =
+                (* binding itself into env for recursion *)
+                Option.fold
+                  ~none: env ~some: (fun v -> Env.bind (v, f) env)
+                  itself
+                |> Env.bind (var, aux a)
+              in
               eval env @@ Apply (e, args)
           end
         | External f ->