Implement replacement and functional updates with lens-law checks

runtime.ml

@@ -4,3 +4,12 @@ let sample : (string * bool, string) lens' =
   lens ~view:fst ~set:(fun value (_, other) -> value, other)
 
 let () = assert (view sample ("colour", true) = "colour")
+
+let () =
+  let source = "colour", true in
+  assert (set sample "grey" source = ("grey", true));
+  assert (set sample (view sample source) source = source);
+  assert (set sample "blue" (set sample "grey" source) = set sample "blue" source);
+  let calls = ref 0 in
+  assert (over sample ~f:(fun text -> incr calls; text ^ "s") source = ("colours", true));
+  assert (!calls = 1)