Handle list head and tail updates with explicit empty-list errors

runtime.ml

@@ -30,3 +30,13 @@ let () =
   assert (set _id "grey" 9 = "grey");
   assert (view (compose _id _1) (9, false) = view _1 (9, false));
   assert (set (compose _2 _id) "grey" (true, 9) = set _2 "grey" (true, 9))
+
+let () =
+  assert (view _hd [1; 2] = 1);
+  assert (view _tl [1] = []);
+  assert (set _hd 3 [1; 2] = [3; 2]);
+  assert (set _tl [3; 4] [1; 2] = [1; 3; 4]);
+  List.iter (fun action ->
+    try action (); assert false with Invalid_argument _ -> ())
+    [(fun () -> ignore (view _hd [])); (fun () -> ignore (view _tl []));
+     (fun () -> ignore (set _hd 1 [])); (fun () -> ignore (set _tl [] []))]