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

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.

Re: with-demoted-errors

[Glenn Morris]

Stefan Monnier wrote:

> I remember having similar desires in other places.  Should we add
> a "format" argument to with-demoted-errors?

Yes please. I've previously needed the same thing in eg
dir-locals-read-from-file.

Re: with-demoted-errors

[Stefan Monnier]

>> I remember having similar desires in other places.  Should we add
>> a "format" argument to with-demoted-errors?
> Yes please. I've previously needed the same thing in eg
> dir-locals-read-from-file.

Installed,


        Stefan

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

[Karl Fogel]

From: Stefan Monnier <monnier@iro.umontreal.ca>
>> +                      ;; 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?

I'm not sure I understand the original comment in saveplace.el,
actually.  Does "mention save-place" mean something special there?

The error string already says "save-place-file", and it will print `err'
too...  In other words, why couldn't one just use `with-demoted-errors'
in that spot right now?  Another way to say it is: the BODY argument to
`with-demoted-errors' can format anything in its error messages that it
want to, so I don't see why that code in saveplace.el isn't using
`with-demoted-errors' already.

Not a facetious question; I'm sure I'm just missing something obvious,
but having pondered I can't think of what it is, so am asking.

-Karl

Re: with-demoted-errors

[Glenn Morris]

Karl Fogel wrote:

> From: Stefan Monnier <monnier@iro.umontreal.ca>
>>> +                      ;; 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?
>
> I'm not sure I understand the original comment in saveplace.el,
> actually.  Does "mention save-place" mean something special there?
>
> The error string already says "save-place-file", and it will print `err'
> too...  In other words, why couldn't one just use `with-demoted-errors'
> in that spot right now?  Another way to say it is: the BODY argument to
> `with-demoted-errors' can format anything in its error messages that it
> want to, so I don't see why that code in saveplace.el isn't using
> `with-demoted-errors' already.

Because

(with-demoted-errors 
   (car (read-from-string
      (buffer-substring (point-min) (point-max)))))

just prints "end-of-file", which gives you no clue that the error is
coming from save-place.

Re: with-demoted-errors

[Karl Fogel]

Glenn Morris <rgm@gnu.org> writes:
>Karl Fogel wrote:
>> From: Stefan Monnier <monnier@iro.umontreal.ca>
>>>> +                      ;; 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?
>>
>> I'm not sure I understand the original comment in saveplace.el,
>> actually.  Does "mention save-place" mean something special there?
>>
>> The error string already says "save-place-file", and it will print `err'
>> too...  In other words, why couldn't one just use `with-demoted-errors'
>> in that spot right now?  Another way to say it is: the BODY argument to
>> `with-demoted-errors' can format anything in its error messages that it
>> want to, so I don't see why that code in saveplace.el isn't using
>> `with-demoted-errors' already.
>
>Because
>
>(with-demoted-errors 
>   (car (read-from-string
>      (buffer-substring (point-min) (point-max)))))
>
>just prints "end-of-file", which gives you no clue that the error is
>coming from save-place.

Oh I see it now; and the proposed solution obviously makes sense.
Thanks.

-K