Subject: bug#37540: [PATCH] Wishlist: Allow edebug-specs to signal arbitrary error strings on syntax errors in macros.

From: Alan Mackenzie <[email-masked]>
Date: Sat, 28 Sep 2019 12:30:34 +0000

Hello, Emacs.

I think it would be a good idea to allow edebug-specs to signal syntax
errors with arbitrary strings, like:

     ("`" &error "Too deeply nested backquotes")

.  The motivation is to complete the correction in bug #31090, which
fixed edebug's handling of nested backquotes in macros.

The problem with that fix is it doesn't handle triply nested backquotes
(without , or ,@ between them) at all well.  There is a fundamental
structure clash between backquote's nesting and Lisp's nesting of
parentheses.

The pragmatic solution is simply to ban triply nested backquotes (i.e.
three `s without ,s or ,@s between them) from edebug instrumentation.
However, this really needs a mechanism to output an error message
string.  There is currently no such mecahanism in edebug.

The following patch implements an &error mechanism in edebug.el, and
illustrates its use in the case which motivated it.

Any objections to committing this patch (together with the needed
amendments to documentation and NEWS)?

diff --git a/lisp/emacs-lisp/edebug.el b/lisp/emacs-lisp/edebug.el
index 717026995a..73257f1568 100644
--- a/lisp/emacs-lisp/edebug.el
+++ b/lisp/emacs-lisp/edebug.el
@@ -1683,6 +1683,7 @@ edebug-match-specs
                 (cl-macrolet-body . edebug-match-cl-macrolet-body)
 		(&not . edebug-match-&not)
 		(&key . edebug-match-&key)
+		(&error . edebug-match-&error)
 		(place . edebug-match-place)
 		(gate . edebug-match-gate)
 		;;   (nil . edebug-match-nil)  not this one - special case it.
@@ -1816,6 +1817,14 @@ edebug-match-&key
                            (car (cdr pair))))
 		 specs))))
 
+(defun edebug-match-&error (cursor specs)
+  ;; Signal an error, using the following string in the spec as argument.
+  (setq edebug-gate t)
+  (let* ((error-string (car specs)))
+    (if (stringp error-string)
+        (edebug-no-match cursor error-string)
+      (error "String expected after &error in edebug-spec"))))
+
 
 (defun edebug-match-gate (_cursor)
   ;; Simply set the gate to prevent backtracking at this level.
@@ -2185,6 +2194,8 @@ backquote-form
 
 (def-edebug-spec nested-backquote-form
   (&or
+   ("`" &error "Triply nested backquotes (without commas \"between\" them) \
+are too difficult to instrument")
    ;; Allow instrumentation of any , or ,@ contained within the (\, ...) or
    ;; (\,@ ...) matched on the next line.
    ([&or "," ",@"] backquote-form)

-- 
Alan Mackenzie (Nuremberg, Germany).