Re: non-local exits with signal and condition-case

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

> This is a heavily simplified version of something I just wrote for the
> new elnode based emacswiki:

Oh, I misunderstood, then: you're simulating "exceptions", which are
called "signals" in Elisp, so you definitely want to use `signal' and
`condition-case' here, rather than catch/throw.

The only inconvenient is that signal names are global, so you need to
use a name of the form "<prefix>-redirect".  I guess you could use an
uninterned signal name, but it might be tricky to make use of that in
a convenient way.

>> You probably want to cons `errv' in front of inherits-list.

> I thought about that before I sent it - I think you are probably
> right.  But my understanding is that you don't need the symbol to be
> present; you can disassociate the symbol used to send the signal from
> the symbol(s) used to capture it.

Yes, if you don't put `errv' in there, indeed you dissociate the two, so
you can't catch an "errv" with "errv" but only with one of its parents.

> I think (as you suggest) a define-signal form should probably not
> support that directly because it seems quite counter intuitive.

Exactly.  While it's technically possible, no signal does that and
I can't think of any situation where someone would want to make use of
such a "feature".  So it's more likely to be a source of bugs and
bug-reports than anything else.

>> We could add it to subr.el (tho I'd call it define-signal).
> Shall I send patches then?

Yes.  Please send them to bug-gnu-emacs@gnu.org.


        Stefan