Re: Attaching context info to an error

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> The question is: where should we put this "context" info.
>>>
>>> One possibility is to do something like the following:
>>>
>>>     (defun load (file ...)
>>>       (handler-bind ((error (lambda (err)
>>>                               (signal 'error-during-load
>>>                                       (cons file err)))))
>>>         ...))
>>
>> I'm just guessing - so the argument ERR of the handler function is not a
>> `condition' object of some kind, in the CL sense? What is it?
>
> No, it's the same kind of objects as are bound to `err` in
> (condition-case err ...), i.e. a cons of (ERROR-NAME . ERROR-DATA).
>
> I don't think it makes sense to have different kinds of objects for
> `handler-bind` and for `condition-case`.

WRT to "collecting dynamic context info". I don't know if the following
is useful, it depends on what the context info is, but maybe it can
serve as an inspiration?

I was reading CMUCL code a bit today. CMUCL (and SBCL) do something 
like the below for handler-bind (and analogously for restart-bind):

  (defvar *handler-clusters* nil)

  (defmacro handler-bind (bindings &body forms)
    "(HANDLER-BIND ( {(type handler)}* )  body)
     Executes body in a dynamic context where the given handler bindings are
     in effect.  Each handler must take the condition being signalled as an
     argument.  The bindings are searched first to last in the event of a
     signalled condition."
    (unless (every #'(lambda (x) (and (listp x) (= (length x) 2))) bindings)
      (simple-program-error (intl:gettext "Ill-formed handler bindings.")))
    `(let ((*handler-clusters*
            (cons (list ,@(mapcar #'(lambda (x) `(cons ',(car x) ,(cadr x)))
                                  bindings))
                  *handler-clusters*)))
       (multiple-value-prog1
        (progn ,@forms)
        ;; Wait for any float exceptions
        #+x87 (float-wait))))

Each handler-bind is associated with a binding of *handler-clusters*,
which contains two things: the handlers of the handler-bind, and a link
to the next such binding up the stack. (This chain is used to find
handlers etc.).

Maybe one could use such a construct in Emacs to additionally associate
context with dynamic occurrances of handler-bind?