add Printexc.exn_slot_id to the runtime

add changelog entry

Changes.md

@@ -25,7 +25,8 @@ Unreleased
   ([#472](https://github.com/melange-re/melange/pull/472))
 - melange: Make `Pervasives` exactly match the `Stdlib` behavior
   ([#476](https://github.com/melange-re/melange/pull/476))
-- melange:
+- melange: fix unbound error when trying to use `Printexc.exn_slot_id`
+  ([#482](https://github.com/melange-re/melange/pull/482))
 
 0.3.2 2022-11-19
 ---------------

jscomp/runtime/caml_exceptions.ml

@@ -78,11 +78,26 @@ let create (str : string) : string =
    This is not a problem in `try .. with` since the logic above is not expressible, see more design in [destruct_exn.md]
 *)
 let caml_is_extension (type a ) (e : a) :  bool  =
-  if Js.testAny e then false 
+  if Js.testAny e then false
   else Js.typeof (Obj.magic e : t) .id = "string"
 
 
 
 
 (**FIXME: remove the trailing `/` *)
 let caml_exn_slot_name (x : t) : string = x.id
+let caml_exn_slot_id : t -> int = [%raw{|function(x){
+  if (x.RE_EXN_ID != null) {
+    var parts = x.RE_EXN_ID.split("/");
+    if (parts.length > 1) {
+      return Number(parts[parts.length - 1])
+    } else {
+      return -1;
+    }
+  } else {
+    return -1;
+  }
+}
+|}]
+
+

test/print-exn.t

@@ -9,6 +9,11 @@ An example that uses exceptions runtime
   >   try f () with
   >   | exn -> Printexc.exn_slot_id exn
   > let result = run (fun _ -> failwith "oops")
+  > 
+  > exception Custom
+  > let () =
+  >   Js.log2 (Printexc.exn_slot_id Custom) (Printexc.exn_slot_id Not_found)
+  > 
   > EOF
   $ cat > dune-project <<EOF
   > (lang dune 3.7)
@@ -21,6 +26,5 @@ An example that uses exceptions runtime
   >  (module_system commonjs))
   > EOF
   $ dune build @melange
-  $ node ./_build/default/melange/x.js 2>&1 | awk '/exn_slot_id/'
-      return Printexc.exn_slot_id(Caml_js_exceptions.internalToOCamlException(raw_exn));
-  TypeError: Printexc.exn_slot_id is not a function
+  $ node ./_build/default/melange/x.js
+  7 -1