diff --git a/examples/id_sexp.ml b/examples/id_sexp.ml
index 90e63c27..cac7b040 100644
--- a/examples/id_sexp.ml
+++ b/examples/id_sexp.ml
@@ -1,13 +1,18 @@
 
-
-let () =
-  if Array.length Sys.argv <> 2 then failwith "usage: id_sexp file";
-  let f = Sys.argv.(1) in
-  let s = CCSexpM.parse_file_list f in
-  match s with
+let pp_sexp s = match s with
   | `Ok l ->
       List.iter
         (fun s -> Format.printf "@[%a@]@." CCSexpM.print s)
         l
   | `Error msg ->
       Format.printf "error: %s@." msg
+
+let () =
+  match Sys.argv with
+  | [| _ |] ->
+      let s = CCSexpM.parse_chan_list stdin in
+      pp_sexp s
+  | [| _; file |] ->
+      let s = CCSexpM.parse_file_list file in
+      pp_sexp s
+  | _ -> failwith "usage: id_sexp [file]"