[Cuis-dev] [Ann] Refinements to Exception handling
Andres Valloud
ten at smallinteger.com
Tue Oct 15 11:04:29 PDT 2019
> So, as far as I understand, you are questioning that people do not
> really know what they do when they do ^5 in an exc. handler and I think
> that they do know. Of course, beginners will make mistakes, but those
> mistakes are the same of using non local returns in other places and
> they are necessary to learn.
Again, I'm arguing from the perspective of the long term consequences of
doing things today, and from the experience of having had to debug and
modify exception frameworks on multiple occasions in the past several years.
Merely being able and competent to do something doesn't mean it should
be done.
Andres.
>
>
> On 10/14/19 04:17, Hernan Wilkinson wrote:
> > I checked and the exception handling mechanism is not affected by a
> > non-local return in an exception handler, it works fine.
> > I think that having a non local return in an exc. handler is really
> > handy and I do not see the problem with that really. Of course that
> > people can make mistakes but that is related to not using non local
> > returns correctly, not to doing it in the exc. handler.
> > The #return:, #pass, etc. do no have the same behavior as having
> a non
> > local return... with this change if I want to return from the method
> > when handling an exception a special object will have to be
> returned and
> > the a check with an if for it will be needed.
> >
> > I think this change is a step backwards really, it makes things
> harder
> > for handler you want to return from the method and having a non
> local
> > return does not harm how exceptions work.
> >
> > Hernan.
> >
> > On Sun, Oct 13, 2019 at 5:12 PM Andres Valloud via Cuis-dev
> > <cuis-dev at lists.cuis.st <mailto:cuis-dev at lists.cuis.st>
> <mailto:cuis-dev at lists.cuis.st <mailto:cuis-dev at lists.cuis.st>>> wrote:
> >
> > The conversation that led to these changes went more or less like
> > this... first, I noted that exception handlers that look like
> this:
> >
> > [:ex | ^5]
> >
> > are asking for trouble, because:
> >
> > a) effectively, nobody is looking at how the exception handling
> > mechanism is implemented, so doing non-local return like that
> curtails
> > the private implementation of exception handling,
> >
> > b) that kind of behavior is what the messages #return,
> #resume, #pass,
> > #outer, etc are there for,
> >
> > c) when you need to debug why exceptions are behaving a
> certain way,
> > the
> > above exception handlers make debugging impossible because
> now there's
> > no place to put a breakpoint (e.g. in #return, #resume,
> #pass, #outer,
> > etc, which can be redefined for the purpose at hand in the single
> > exception of interest), and finally,
> >
> > d) it's possible to distort the exception handling mechanism
> behavior
> > with non-local returns.
> >
> >
> > The vast majority of the time, I see exception handlers with
> non-local
> > returns as very close to expressions such as these:
> >
> > #(1 2 3) asSortedCollection: [:x :y | ^x < y]
> >
> > where the behavior you get is not what you expected, even
> though the
> > code is doing exactly what was requested. See attached for
> an actual
> > example of what I mean. To run the code, do these and see
> what happens.
> >
> > ExceptionExample new breakingStuff
> > ExceptionExample new workingStuff
> >
> > To me, generally speaking, these considerations push the
> scales against
> > the apparent convenience of having exception handlers do a
> non-local
> > return. I do not want to have to debug that kind of
> convenience under
> > time pressure. I do not even want to have to change code
> that uses
> > such
> > convenience, because it's going to be more difficult than
> otherwise.
> >
> >
> > So Juan heard this and pointed out the reasoning is lovely
> but it's not
> > that helpful because the system is doing nothing to make
> users aware of
> > these facts. He suggested the system should complain
> instead. Thus we
> > (although he says mostly I) wrote code that detects non-local
> return
> > and
> > complains. I believe that code was integrated recently (this
> bit, he
> > did on his own LOL).
> >
> > Andres.
> >
> > On 10/13/19 07:52, Hernan Wilkinson wrote:
> > > Could you provide the case were it generates problems?
> > >
> > > On Sun, Oct 13, 2019 at 11:40 AM Hernan Wilkinson
> > > <hernan.wilkinson at 10pines.com
> <mailto:hernan.wilkinson at 10pines.com>
> > <mailto:hernan.wilkinson at 10pines.com
> <mailto:hernan.wilkinson at 10pines.com>>
> > <mailto:hernan.wilkinson at 10pines.com
> <mailto:hernan.wilkinson at 10pines.com>
> > <mailto:hernan.wilkinson at 10pines.com
> <mailto:hernan.wilkinson at 10pines.com>>>> wrote:
> > >
> > > Hi,
> > > I think that forbidden a non local return in an
> exception
> > handler
> > > is not really the solution... Although I understand the
> > motivation I
> > > think that changing the exception handling mechanism
> would be
> > better.
> > > I'm going to take a look at it to see if it is
> possible to
> > do it.
> > >
> > > Hernan.
> > >
> > > On Sun, Oct 13, 2019 at 10:48 AM Juan Vuletich via
> Cuis-dev
> > > <cuis-dev at lists.cuis.st
> <mailto:cuis-dev at lists.cuis.st> <mailto:cuis-dev at lists.cuis.st
> <mailto:cuis-dev at lists.cuis.st>>
> > <mailto:cuis-dev at lists.cuis.st
> <mailto:cuis-dev at lists.cuis.st> <mailto:cuis-dev at lists.cuis.st
> <mailto:cuis-dev at lists.cuis.st>>>> wrote:
> > >
> > > Hi Folks,
> > >
> > > Some time ago, Andrés (with just a tad of help
> from me)
> > fixed a
> > > problem
> > > in Exception handling. If exception handler blocks do
> > non-local
> > > return
> > > (^stuff), they will skip execution of part of the
> Exception
> > > handling
> > > system code, breaking exception return values and
> > possibly other
> > > 'bad
> > > things'.
> > >
> > > I just pushed to GitHub a few updates with this work.
> > Now, if an
> > > exception handler does a non-local return, an
> Error will be
> > > raised. We
> > > also fixed a couple of places in the image where
> this was
> > being
> > > done.
> > > BaseImageTests pass.
> > >
> > > This updates have some risk of breaking your code.
> If you you
> > > get this
> > > error: 'Exception handler blocks must not do non local
> > returns',
> > > then
> > > you need to adjust your code. See updates #3917 to
> #3922
> > for the
> > > changes
> > > done to the base image.
> > >
> > > Thanks,
> > >
> > > --
> > > Juan Vuletich
> > > www.cuis-smalltalk.org <http://www.cuis-smalltalk.org>
> <http://www.cuis-smalltalk.org>
> > <http://www.cuis-smalltalk.org>
> > > https://github.com/Cuis-Smalltalk/Cuis-Smalltalk-Dev
> > > https://github.com/jvuletich
> > > https://www.linkedin.com/in/juan-vuletich-75611b3
> > > @JuanVuletich
> > >
> > > --
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> > > Cuis-dev at lists.cuis.st <mailto:Cuis-dev at lists.cuis.st>
> <mailto:Cuis-dev at lists.cuis.st <mailto:Cuis-dev at lists.cuis.st>>
> > <mailto:Cuis-dev at lists.cuis.st
> <mailto:Cuis-dev at lists.cuis.st> <mailto:Cuis-dev at lists.cuis.st
> <mailto:Cuis-dev at lists.cuis.st>>>
> > > https://lists.cuis.st/mailman/listinfo/cuis-dev
> > >
> > >
> > >
> > > --
> > > *Hernán Wilkinson
> > > Agile Software Development, Teaching & Coaching*
> > > *Phone: +54-011*-4893-2057
> > > *Twitter: @HernanWilkinson*
> > > *site: http://www.10Pines.com <http://www.10pines.com/>*
> > > Address: Alem 896, Floor 6, Buenos Aires, Argentina
> > >
> > >
> > >
> > > --
> > > *Hernán Wilkinson
> > > Agile Software Development, Teaching & Coaching*
> > > *Phone: +54-011*-4893-2057
> > > *Twitter: @HernanWilkinson*
> > > *site: http://www.10Pines.com <http://www.10pines.com/>*
> > > Address: Alem 896, Floor 6, Buenos Aires, Argentina
> > --
> > Cuis-dev mailing list
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> > https://lists.cuis.st/mailman/listinfo/cuis-dev
> >
> >
> >
> > --
> > *Hernán Wilkinson
> > Agile Software Development, Teaching & Coaching*
> > *Phone: +54-011*-4893-2057
> > *Twitter: @HernanWilkinson*
> > *site: http://www.10Pines.com <http://www.10pines.com/>*
> > Address: Alem 896, Floor 6, Buenos Aires, Argentina
> --
> Cuis-dev mailing list
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>
>
>
> --
> *Hernán Wilkinson
> Agile Software Development, Teaching & Coaching*
> *Phone: +54-011*-4893-2057
> *Twitter: @HernanWilkinson*
> *site: http://www.10Pines.com <http://www.10pines.com/>*
> Address: Alem 896, Floor 6, Buenos Aires, Argentina
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