Misc: improve doc for Url.decode_arguments (#1019)

Author: hhugo

lib/js_of_ocaml/url.mli

@@ -77,7 +77,8 @@ val encode_arguments : (string * string) list -> string
 
 val decode_arguments : string -> (string * string) list
 (** [decode_arguments s] parses [s] returning the sliced-diced
-    association list. *)
+    association list. [s] should be only the arguments part (after the '?')
+    not the whole url. *)
 
 (** The following part allow one to handle Url object in a much higher level
     than what a string provides. *)

lib/tests/test_url.ml

@@ -39,3 +39,20 @@ let%expect_test _ =
   then ()
   else print_endline "escaping error";
   [%expect {||}]
+
+let%expect_test "[decode_arguments]" =
+  let test url =
+    ListLabels.iter (Url.decode_arguments url) ~f:(fun (key, value) ->
+        Printf.printf "'%s' => '%s'\n" key value)
+  in
+  (* Incorrect usage: passing in full url. *)
+  test "https://foo.com/?foo=bar&baz%20=qux%22quux";
+  [%expect {|
+    'https://foo.com/?foo' => 'bar'
+    'baz ' => 'qux"quux' |}];
+  (* correct usage: only the arguments part *)
+  test "foo=bar&baz%20=qux%22quux";
+  [%expect {|
+    'foo' => 'bar'
+    'baz ' => 'qux"quux' |}];
+  ignore ()