Re: [TCLCORE] =?iso-8859-15?q?TIP_=23290=3A_Registration_of_Custom_Er?= =?iso-8859-15?q?ror_Handler_Scripts?=

[Eckhard L. <ec...@we...>]

> -----Urspr=FCngliche Nachricht-----
> Von: Lars Hellstr=F6m <Lar...@re...>
> Gesendet: 01.11.06 21:10:29
> An: tcl...@li...
> Betreff: Re: [TCLCORE] TIP #290: Registration of Custom Error Handler Sc=
ripts


> 2006-11-01 kl. 15.02 skrev Eckhard Lehmann:
> >  This TIP proposes the possibility to register custom scripts or
> >  commands in the usual Tcl event handler style as error handlers.
>=20
> Does "event" here have anything to do with the event loop=3F

No, it has not. It's just the same "format" like commands for fileevent, w=
indow event and so on.

>=20
> >  SPECIFICATION
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> [...]
>=20
> IMHO, this is not a specification, but rather a ChangeLog.

I know.. sorry, I did'nt TIP before ;-). Its updated and hopefully better =
now.

> Attempting to recreate a functionality description from the information=20
> given, it seems like these error handlers would be called immediately=20
> when the error is thrown, just like e.g. a variable trace is called=20
> immediately when that variable is accessed. In view of that, it might=20
> perhaps be more appropriate to fully integrate this into the [trace]=20
> command (especially since this would provide established mechanisms for=20
> introspection and handler deletion).

I thought about a [trace set errorcmd] or [trace add errorcmd]. Probably y=
ou're right regarding the interface to this from a Tcl'ers point of view.=20
But the internal execution mechanism is different... it's not a "normal" t=
race, which runs when all the other traces are executed, but only when the=
re was an error. In fact, it is called after the command execution, if nec=
essary as determined by the catch level and -caught/-uncaught flags.

> Come to think of it, how much of this isn't already made possible by=20
> [trace add execution]=3F If you do [trace add execution main leavestep=20
> handler], where [main] is the main procedure of your program and=20
> [handler] is your designated error handler, then this handler will=20
> indeed be called whenever a command completes, and thus be given a=20
> chance to do something for those returns that have an error return=20
> code.=20

I made a section in the TIP explaining my experiences with that. Indeed I =
thought it could be solved this way, but it was somewhat tedious in practi=
ce...

> (Even counting [catch]es could be handled using execution traces,=20
> if one trusts the user not to rename the [catch] command.) Speed is of=20
> course an issue, but if that is the main reason for this feature then=20
> the TIP should say so.

As for the [catch]'es - I don't know how to capture them in a leavestep tr=
ace. Its not obvious or easy, if possible at all. The speed issue is inclu=
ded in the TIP now.

>=20
> The distinction between caught and uncaught errors also seem a bit=20
> hazy. Many errors are caught (by control structures and the like) just=20
> to be rethrown, but it seems as the mechanism proposed here would=20
> simply consider them to be caught.=20

No, if they are rethrown, a new error is generated which is not considered=
 to be caught (if not caught elsewhere above, of course).

> That is probably not what a user=20
> would expect, and a control structure implementor would want a=20
> mechanism for saying "this [catch] is not to count as catching errors".

The custom command can be run on either caught errors or uncaught errors, =
or both (depending on the -caught/-uncaught flags given to the registratio=
n command). Everything else is up to the implementor of the registered com=
mand (checking ::errorCode and ::errorInfo etc.). I think that is appropri=
ate...


Eckhard

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