Implement replacement and functional updates with lens-law checks

fieldglass.ml

@@ -3,3 +3,5 @@ type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens
 
 let lens ~view ~set = view, set
 let view (get, _) = get
+let set (_, put) = put
+let over (get, put) ~f source = put (f (get source)) source

fieldglass.mli

@@ -3,3 +3,5 @@ type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens
 
 val lens : view:('s -> 'a) -> set:('b -> 's -> 't) -> ('s, 't, 'a, 'b) lens
 val view : ('s, 't, 'a, 'b) lens -> 's -> 'a
+val set : ('s, 't, 'a, 'b) lens -> 'b -> 's -> 't
+val over : ('s, 't, 'a, 'b) lens -> f:('a -> 'b) -> 's -> 't

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)