Re: Attaching context info to an error

[João Távora]

On Fri, Dec 29, 2023 at 2:55 AM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>
> > What object exactly goes into the handler bind handlers?
>
> The same (even `eq`) as the one passed to the `condition-case` handler.

Would it matter if it weren't?

> > Is it the funny cons?
>
> Yup.  That's ELisp's current notion of an "error object".
>
> > Can we at least get some generic functions to hide that representation
> > so that we can change it later on?
>
> Patch very welcome, yes (and this is orthogonal to `handler-bind`).

What operations are typically requested from it?

- Printing?  a cl-print-object would be sufficient, I think,
  along with a (orthogonal) directive to 'format' for using
  cl-prin1 (why doesn't cl-princ exist?)

- Getting the type for resignalling purposes?  Unfortunately,
  this returns 'cons'

     (condition-case err (error "bla") (error (type-of err)))

  Can it be made to return error?

- Can we import a typep (similar to cl-typep) that understands
  basic types EIEIO and also error hierarchy?

- simple-condition-format-control and
  simple-condition-format-arguments?

- change manual to explain that resignalling is discouraged now
  that handler-bind exists, but if resignalling _is_ done, then
  definitely avoid car and cdr and some form of

    (signal (error-symbol err) (error-data err)

  introducing these new functions?

- go through the hierarchy and add <error-name>-<data-slot>
  accessors? Or do we already consider these details of "data" to
  to be hidden?  If so, I think surely some violations exist,
  hopefully not many.

  They shouldn't be terribly hard to find.  From some greps I
  estimate there less than 700 condition-case that actually use
  the variable and the majority seems to be resignalling or
  printing.  The former will probably be fixed by
  handler-bind anyway, and printing is a generic operation
  already.  I checked about 20 occurances randomly, and they
  all fell into this basket.  This also tells me that switching
  to proper objects isn't that hard.

João