with-demoted-errors (was: [Emacs-diffs] trunk r114172: * lisp/saveplace.el (load-save-place-alist-from-file): Demote errors.)

[Stefan Monnier]

> +                      ;; This is with-demoted-errors, but we want to
> +                      ;; mention save-place in any error message.
> +                      (condition-case err
> +                        (car (read-from-string
> +                              (buffer-substring (point-min) (point-max))))
> +                        (error (message "Error reading save-place-file: %S" 
> err)
> +                               nil)))
 
I remember having similar desires in other places.  Should we add
a "format" argument to with-demoted-errors?


        Stefan


=== modified file 'lisp/subr.el'
--- lisp/subr.el        2013-09-05 03:46:34 +0000
+++ lisp/subr.el        2013-09-09 17:58:01 +0000
@@ -3350,16 +3350,22 @@
 (define-obsolete-function-alias 'condition-case-no-debug
   'condition-case-unless-debug "24.1")
 
-(defmacro with-demoted-errors (&rest body)
+(defmacro with-demoted-errors (format &rest body)
   "Run BODY and demote any errors to simple messages.
 If `debug-on-error' is non-nil, run BODY without catching its errors.
 This is to be used around code which is not expected to signal an error
-but which should be robust in the unexpected case that an error is signaled."
-  (declare (debug t) (indent 0))
-  (let ((err (make-symbol "err")))
+but which should be robust in the unexpected case that an error is signaled.
+For backward compatibility, if FORMAT is not a constant string, it
+is assumed to be part of BODY, in which case the message format
+used is \"Error: %S\"."
+  (declare (debug t) (indent 1))
+  (let ((err (make-symbol "err"))
+        (format (if (and (stringp format) body) format
+                  (prog1 "Error: %S"
+                    (if format (push format body))))))
     `(condition-case-unless-debug ,err
-         (progn ,@body)
-       (error (message "Error: %S" ,err) nil))))
+         ,(macroexp-progn body)
+       (error (message ,format ,err) nil))))
 
 (defmacro combine-after-change-calls (&rest body)
   "Execute BODY, but don't call the after-change functions till the end.