Add polymorphic tuple projections and replacements

fieldglass.ml

@@ -9,3 +9,6 @@ let over (get, put) ~f source = put (f (get source)) source
 let compose (outer_get, outer_put) (inner_get, inner_put) =
   (fun source -> inner_get (outer_get source)),
   (fun value source -> outer_put (inner_put value (outer_get source)) source)
+
+let _1 = (fun (left, _) -> left), (fun left (_, right) -> left, right)
+let _2 = (fun (_, right) -> right), (fun right (left, _) -> left, right)

fieldglass.mli

@@ -6,3 +6,5 @@ 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
 val compose : ('s, 't, 'a, 'b) lens -> ('a, 'b, 'c, 'd) lens -> ('s, 't, 'c, 'd) lens
+val _1 : ('a * 'c, 'b * 'c, 'a, 'b) lens
+val _2 : ('c * 'a, 'c * 'b, 'a, 'b) lens

runtime.ml

@@ -20,3 +20,8 @@ let () =
   let focus = compose first second in
   assert (view focus ((true, "grey"), false) = "grey");
   assert (set focus 9 ((true, "grey"), false) = ((true, 9), false))
+
+let () =
+  assert (set _1 "grey" (1, true) = ("grey", true));
+  assert (over _2 ~f:string_of_int (false, 9) = (false, "9"));
+  assert (view (compose _1 _2) ((true, "grey"), ()) = "grey")