Lisp-friendly backtraces [was: Lispy backtraces]

["Clément Pit--Claudel" <clement.pit@gmail.com>]

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On 2016-12-04 22:30, Eli Zaretskii wrote:
>>> So would it perhaps make sense to rename 'backtrace' into something
>>> like 'backtrace--internal', and make it accept one more argument, the
>>> function to apply to each frame, which is now hard-coded as 'prin1'?
>>> Would that allow you to implement 'backtrace' in Lisp and also
>>> implement whatever application you had in mind, by calling
>>> 'backtrace--internal' passing it your own function instead of 'prin1'?
>>
>> Quite possibly!
>
> Will you be working on this idea at some point?

Yup.  I've attached a rough patch, and an ELisp file showing what calling the new function (mapbacktrace) looks like.

Let me know if this direction makes sense.  If so, I will write the corresponding documentation, a Changelog entry, and a proper commit message.

(CC Alan, since you recently expressed frustration with the contents of the *Backtrace* buffer; hopefully this new function will help with implementing a more flexible *Backtrace* buffer?)

Cheers,
Clément.


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From 861c841eb242c474c66421ce4d9964940033ff31 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20Pit--Claudel?= <clement.pitclaudel@live.com>
Date: Mon, 5 Dec 2016 00:52:14 -0500
Subject: [PATCH] New function mapbacktrace

---
 src/eval.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 49 insertions(+), 1 deletion(-)

diff --git a/src/eval.c b/src/eval.c
index 724f001..dcda51c 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3420,6 +3420,53 @@ The debugger is entered when that frame exits, if the flag is non-nil.  */)
   return flag;
 }
 
+DEFUN ("mapbacktrace", Fmapbacktrace, Smapbacktrace, 1, 2, 0,
+       doc: /* Call FUNCTION for each frame in backtrace.
+FUNCTION is called with 4 arguments EVALD FUNC ARGS FLAGS.  If a frame
+has not evaluated its arguments yet or is a special form, EVALD is nil
+and ARGS is a list of forms.  If a frame has evaluated its arguments
+and called its function already, EVALD is t and ARGS is a list of
+values.  FLAGS is a plist of properties of the current frame:
+currently, the only supported property is :debug-on-exit.
+If NSKIP is non-nil, the top NSKIP frames are skipped.
+`mapbacktrace' always returns nil.  */)
+     (Lisp_Object function, Lisp_Object nskip)
+{
+  union specbinding *pdl = backtrace_top ();
+
+  if (!NILP (nskip))
+    {
+      CHECK_NUMBER(nskip);
+      EMACS_INT to_skip = XINT(nskip);
+      while (to_skip > 0 && backtrace_p (pdl)) {
+        to_skip--;
+        pdl = backtrace_next (pdl);
+      }
+    }
+
+  while (backtrace_p (pdl))
+    {
+      Lisp_Object flags = Qnil;
+      if (backtrace_debug_on_exit (pdl))
+        {
+          flags = Fcons (QCdebug_on_exit, Fcons (Qt, Qnil));
+        }
+
+      if (backtrace_nargs (pdl) == UNEVALLED)
+        {
+          call4 (function, Qnil, backtrace_function (pdl), *backtrace_args (pdl), flags);
+        }
+      else
+        {
+          Lisp_Object tem = Flist (backtrace_nargs (pdl), backtrace_args (pdl));
+          call4 (function, Qt, backtrace_function (pdl), tem, flags);
+        }
+      pdl = backtrace_next (pdl);
+    }
+
+  return Qnil;
+}
+
 DEFUN ("backtrace", Fbacktrace, Sbacktrace, 0, 0, "",
        doc: /* Print a trace of Lisp function calls currently active.
 Output stream used is value of `standard-output'.  */)
@@ -3973,7 +4020,8 @@ alist of active lexical bindings.  */);
   defsubr (&Srun_hook_wrapped);
   defsubr (&Sfetch_bytecode);
   defsubr (&Sbacktrace_debug);
-  defsubr (&Sbacktrace);
+  DEFSYM (QCdebug_on_exit, ":debug-on-exit");
+  defsubr (&Smapbacktrace);
   defsubr (&Sbacktrace_frame);
   defsubr (&Sbacktrace_eval);
   defsubr (&Sbacktrace__locals);
-- 
2.7.4


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;; -*- lexical-binding: t -*-

(defun backtrace-1 (evald func args flags)
  "Print a trace of a single stack frame to `standard-output'.
EVALD, FUNC, ARGS, FLAGS are as in `mapbacktrace'."
  (let ((print-level (or print-level 8)))
    (princ (if (plist-get flags :debug-on-exit) "* " "  "))
    (cond
     ((and evald (not debugger-stack-frame-as-list))
      (prin1 func)
      (if args (prin1 args) (princ "()")))
     (t
      (prin1 (cons func args))))
    (princ "\n")))

(defun backtrace ()
  "Print a trace of Lisp function calls currently active.
Output stream used is value of `standard-output'."
  (mapbacktrace #'~/backtrace-1 1))

(defun backtrace-frames ()
  "Collect all frames of current backtrace into a list."
  (let ((frames nil))
    (mapbacktrace (lambda (&rest frame) (push frame frames)) 2)
    (nreverse frames)))

(defun ~/backtrace-frame (nframes &optional base)
  "Return the function and arguments NFRAMES up from current execution point.
If that frame has not evaluated the arguments yet (or is a special form),
the value is (nil FUNCTION ARG-FORMS...).
If that frame has evaluated its arguments and called its function already,
the value is (t FUNCTION ARG-VALUES...).
A &rest arg is represented as the tail of the list ARG-VALUES.
FUNCTION is whatever was supplied as car of evaluated list,
or a lambda expression for macro calls.
If NFRAMES is more than the number of frames, the value is nil.
If BASE is non-nil, it should be a function and NFRAMES counts from its
nearest activation frame."
  (let ((frame nil))
    (mapbacktrace (lambda (evald func args _)
                    (when (and base (eq func base))
                      (setq base nil))
                    (unless base
                      (when (eq nframes 0)
                        (setq frame `(,evald ,func ,@args)))
                      (setq nframes (1- nframes)))))
    frame))

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