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bug#37540: Bug 37540: [PATCH] Wishlist: Allow edebug-specs to signal arb

From: Alan Mackenzie
Subject: bug#37540: Bug 37540: [PATCH] Wishlist: Allow edebug-specs to signal arbitrary error strings on syntax errors in macros.
Date: Tue, 14 Jan 2020 20:14:13 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

Hello Emacs and Eli.

On Sat, Sep 28, 2019 at 12:30:34 +0000, Alan Mackenzie wrote:
> 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)?

, to which Eli's response was to request me to wait until the emacs-27
release branch had been cut.  That has now happened.

So, are there any objections to me now committing this patch (current
version below, including a documentation amendment), plus a NEWS item,
to the master branch?

diff --git a/doc/lispref/edebug.texi b/doc/lispref/edebug.texi
index 8be8307c75..cfef5c12d1 100644
--- a/doc/lispref/edebug.texi
+++ b/doc/lispref/edebug.texi
@@ -1362,6 +1362,11 @@ Specification List
 is primarily used to generate more specific syntax error messages.  See
 @ref{Backtracking}, for more details.  Also see the @code{let} example.
+@item &error
+@code{&error} should be followed by a string, an error message, in the
+edebug-spec; it aborts the instrumentation, displaying the message in
+the minibuffer.
 @item @var{other-symbol}
 @cindex indirect specifications
 Any other symbol in a specification list may be a predicate or an
diff --git a/lisp/emacs-lisp/edebug.el b/lisp/emacs-lisp/edebug.el
index b8d2fb5beb..a57c48f1f9 100644
--- a/lisp/emacs-lisp/edebug.el
+++ b/lisp/emacs-lisp/edebug.el
@@ -1714,6 +1714,7 @@ 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.
@@ -1847,6 +1848,14 @@ edebug-match-&key
                            (car (cdr pair))))
+(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.
@@ -2216,6 +2225,8 @@ backquote-form
 (def-edebug-spec nested-backquote-form
+   ("`" &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).

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