Add Js_exn.unsafeAnyToExn function

Author: ryyppy

jscomp/others/js_exn.ml

@@ -45,6 +45,12 @@ external makeError : string -> error = "Error" [@@bs.new]
 external isCamlExceptionOrOpenVariant : 
   'a -> bool = "caml_is_extension"
 
+let unsafeAnyToExn obj =
+  if isCamlExceptionOrOpenVariant obj then
+    (Obj.magic obj : exn)
+  else
+    Error ((Obj.magic obj) : t)
+
 let raiseError str = 
   raise (Obj.magic (makeError str : error) : exn)
 

jscomp/others/js_exn.mli

@@ -43,6 +43,25 @@ external isCamlExceptionOrOpenVariant:
   'a -> bool = "caml_is_extension"
 (** internal use only *)
 
+val unsafeAnyToExn : 'a -> exn
+(** [unsafeAnyToExn obj] will take any value [obj] and wrap it
+ * in a Js.Exn.Error if given value is not an exn already. If
+ * [obj] is an exn, it will return [obj] without any changes. 
+ *
+ * This function is mostly useful for cases where you want to unify a type of a value
+ * that potentially is either exn, a JS error, or any other JS value really (e.g. for 
+ * a value passed to a Promise.catch callback) 
+ *
+ * @example {[
+ *   switch (Js.Exn.unsafeAnyToExn("test")) {
+ *     | Js.Exn.Error(v) =>
+ *       switch(Js.Exn.message(v)) {
+ *         | Some(str) => Js.log("We won't end up here")
+           | None => Js.log2("We will land here: ", v)
+ *       }
+ *   }
+ * ]}
+ * **)
 
 (** Raise Js exception Error object with stacktrace *)
 val raiseError : string -> 'a