Re: Attaching context info to an error

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> I'm playing with `handler-bind` and trying to see how we could make use
>> of such a functionality in Emacs.
> `handler-bind' seems to have the main advantage over `condition-case'
> that you get the full information about the original error stack, while
> in a `condition-case' that information is lost, correct?

Yes, it's a kind of mix between `condition-case` and `signal-hook-function`.

>> but I don't like the idea of changing one kind of error into another.
> Why not?  Wrapping errors that way is what I know.

Because a (condition-case err (FOO) (wrong-number-of-arguments ...))
won't catch an `error-during-load` even if that error is wrapped around
a `wrong-number-of-arguments` error.

>> Any other idea how/where we could attach that info?
> Property list of the error-symbol?

That's storing the info in a global var, which is hackish: as you say it
will sometimes "survive the lifetime of the error" but it will also
sometimes live a too short life if some other error happens between the
moment the first error is signaled and that first error is caught.
[ This is likely to occur if we run the debugger between those two, for
  instance, or if an unwind form signal an error, ...  ]

FWIW, a global variable is what we use for the only such "error context
information" we currently have, which is stored in
`Vsignalling_function` (not exposed to ELisp, but visible sometimes in
*Messages*).


        Stefan