New user option 'debugger-stack-frame-as-list'

* src/eval.c (syms_of_eval) <debugger-stack-frame-as-list>: New
variable.
* lisp/cus-start.el (standard): Add debugger-stack-frame-as-list.
* lisp/emacs-lisp/debug.el (debugger-setup-buffer): Adjust
backtrace processing for the value of debugger-stack-frame-as-list.
* lisp/emacs-lisp/edebug.el (edebug-backtrace): Adjust backtrace
processing for the value of debugger-stack-frame-as-list.
* doc/lispref/debugging.texi (Internals of Debugger): Document
debugger-stack-frame-as-list.
* etc/NEWS: Mention 'debugger-stack-frame-as-list'.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/eval.c b/src/eval.c
index e08a25a31a0..407561082d1 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3421,13 +3421,17 @@ Output stream used is value of `standard-output'.  */)
       else
         {
           tem = backtrace_function (pdl);
+          if (debugger_stack_frame_as_list)
+            write_string ("(");
           Fprin1 (tem, Qnil);   /* This can QUIT.  */
-          write_string ("(");
+          if (!debugger_stack_frame_as_list)
+            write_string ("(");
           {
             ptrdiff_t i;
             for (i = 0; i < backtrace_nargs (pdl); i++)
               {
-                if (i) write_string (" ");
+                if (i || debugger_stack_frame_as_list)
+                  write_string(" ");
                 Fprin1 (backtrace_args (pdl)[i], Qnil);
               }
           }
@@ -3850,6 +3854,10 @@ This is nil when the debugger is called under circumstances where it
 might not be safe to continue.  */);
   debugger_may_continue = 1;
 
+  DEFVAR_BOOL ("debugger-stack-frame-as-list", debugger_stack_frame_as_list,
+               doc: /* Non-nil means display call stack frames as lists. */);
+  debugger_stack_frame_as_list = 0;
+
   DEFVAR_LISP ("debugger", Vdebugger,
                doc: /* Function to call to invoke debugger.
 If due to frame exit, args are `exit' and the value being returned;