Compose lenses whilst preserving unfocussed data

fieldglass.ml

@@ -5,3 +5,7 @@ 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
+
+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)