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Re: [Info-gnuprologjava] Adding hooks for executing code when frames are


From: Matt Lilley
Subject: Re: [Info-gnuprologjava] Adding hooks for executing code when frames are being discarded
Date: Fri, 02 Mar 2012 07:56:30 +1300

Hello,

I've attached a patch that fixes three problems with my original submission (and some test cases to cover those, where possible: Because the cleanup doesn't allow exceptions to be thrown, I don't think it's possible to use the test harness to check at least one case, since the harness relies on checking the state after the test has completed, whereas we are interested in state which is immediately backtracked over and lost...)

The first is that setup_call_catcher_cleanup/4 should not succeed if the goal fails, unifying the Catcher to fail; it should unify the Catcher to fail, call the cleanup, and then *fail*. The second, possibly more important fix, is that when backtracking into the goal, we should not call the cleanup at all - only when backtracking *out* of the goal because there are no more solutions. To do this, I changed Interpreter.popBacktrackInfo() to skip over the cleanup frames without cleaning them up. This certainly works, but I wonder if there might be other places that need to also do this. Finally, when running the cleanup, we need to make sure that afterwards the stack looks exactly like it did before the cleanup. To do this I peek at the top frame on the stack and pop back to it after running the cleanup.

Testing this in my application, it does a much better job than before - I should've tested more thoroughly before the original submission!

Thanks again,
Matt
Sorry for the months of delay. This has now been pushed with only minor
changes.
I decided against the throwing of PrologException in cleanup as this was
resulting in compile errors in several other places which needed to
handle this so for now I think it better to ignore exceptions in the
cleanup method. If it is important that they do get thrown then I can
reconsider that.
The code also got reformatted as I have eclipse set to vigorously
enforce GNU's code formatting guidelines.
I moved the unit test you added into extra as it is not testing a core
feature.

Do you want me to spin up a 0.2.x branch release? Or let this form part
of the 0.3.0 release (which won't be until the autumn at the very
earliest)?

Thank you,

Daniel

On Fri, 2011-12-16 at 11:56 +1300, Matt Lilley wrote:
Hello,

Sorry for the confusion; I had intended
Predicate_setup_call_catcher_cleanup.java as an illustration of what
could be achieved by adding cleanup information to the stack; I hadn't
intended it to be included in the main codebase.

Nonetheless, I've tidied it up significantly and adjusted the indent and
coding style to more closely match yours. I've included a patch as a
diff against 0.3.0 (Yes, I'd originally written it against 0.2.6). I
made a slight modification to the way it's implemented as well;
essentially keeping some extra backtrack frames with just the cleanup
info in them. This avoids having to pop off a frame when we detect
there's a choicepoint and push a modified frame; instead we just push an
extra frame which has no effect if we undo it.

I've also included a few tests after reverse engineering the testing
format. Hopefully I guessed correctly. I should point out that even with
a brand new clone of the git repository, some 26 tests fail, and I get
predicate undef_pred/0 does not exist.
predicate ^/2 does not exist.
predicate ^/2 does not exist.
printed to stderr. Is that expected?

Let me know if you have any other suggestions for improvements to the
feature.

Regards,

Matt
Hello,

To add tests put something suitable into the inria suite in
test/inriasuite.
They can be run with:
gnu.prolog.test.GoalRunner --once inriasuite.pl run_all_tests


Predicate_setup_call_catcher_cleanup wants a copyright header and a
package. It also needs to be wired in via an entry in a .pro in
gnu.prolog.vm.buildins. I would prefer it in a patch rather than as the
single file as a patch has metadata about where it should go.

Which version of gnuprologjava did you write
Predicate_setup_call_catcher_cleanup? 0.2.6? It isn't current head
anyway - which doesn't matter I can do a 0.2.7 release with this in and
port the code to the current newmaster.

In terms of imports I prefer to avoid using foo.bar.* as when later I do
Ctrl-Shift-o in eclipse they will get expanded.

When using stuff from args[] if you are only using a particular index
once then it is ok to not assign it to a variable with a meaningful name
but when using a particular index more than once it is much more
readable for them to be named by assigning to a variable at the
beginning. Probably also good to check that that args[] is the right
length (though with correct wiring up it should always get the right
number of arguments).

When ignoring Exceptions a comment is required explaining why this is
sensible. The correct thing to do in this case would probably be to log
this at info or warning but we don't have a logging framework a TODO
would be helpful for when I add that in later.


BacktrackInfoWithCleanup needs its copyright header updating as it
doesn't have you on it and if should only have Constantine on it if you
are reusing code that Constantine wrote.
Similarly a TODO about logging the exception would be helpful.

With those addressed and tests to prove it works and to prevent me
breaking it then that should be fine.

Thank you,

Daniel

On Thu, 2011-12-15 at 11:18 +1300, Matt Lilley wrote:
Hello,

I've attached a patch that does the trick. I don't think it's too
inefficient, though it does make popping frames a bit slower (I don't
think there's any way you can avoid that if the task at hand is to add
complexity to that very process). Also attached is an implementation of
Predicate_setup_call_catcher_cleanup, the more general case of
setup_call_cleanup/3. I'm not entirely sure how your testing framework
works, but I can provide a prolog file as well if that would help.

I'm happy to rework this if you have suggestions on how it could be
better integrated; I'm obviously very new to your project!

Thanks again,
Matt

Hello,

If you want to submit a patch to add that functionality then that would
be great, I have no problem with adding non-ISO features as long as ISO
features don't get broken and as long as it can be implemented cleanly.
Unfortunately I don't have time to work out how to write such a patch
myself at the moment but I do have time to review it. If you could add
some test cases to prove it works that would be great.

If implementing it cleanly is not possible then we can look at what
could be changed to either make that possible or to allow a local hack.

If you have any questions then please ask.

Best regards,

Daniel





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Matt Lilley
Software Engineer
SecuritEase

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