[Info-gnuprologjava] Adding hooks for executing code when frames are being discarded

[Matt Lilley]

Hello,

I'm trying to implement setup_call_cleanup/3 [1] in GNU Prolog for Java 
so that I can port some code from SWI Prolog to GNU Prolog for Java.

For most cases, this is quite simple, but the case where the called goal 
succeeds leaving backtrack points isn't possible at present (I think). 
If you can see why, feel free to skip the next paragraph.

Consider setup_call_cleanup(true, member(A, [a,b,c]), throw(foo)), !. 
After executing member(A, [a,b,c]), the interpreter pushes a backtrack 
point. Then the ICut VM instruction is executed, which simply pops the 
backtrack point off again, and then execution (of this goal, anyway) 
stops - and throw(foo) isn't ever run. In order to execute the 
throw(foo), I need to overload Interpreter.popBacktrackInfo (and similar 
methods) to check for the cleanup-type goals and run them then. Adding 
in an extra backtrack point won't work either, since we never actually 
undo anything - we simply discard frames.

The simplest option from my perspective is to make Interpreter non-final 
and make the constructor public. That would allow me to create my own 
derived interpreter which could include such changes without causing 
problems for anyone else. The second alternative would be to write code 
to handle this and provide it back as a new standard feature of GNU 
Prolog for Java, though since the predicate isn't ISO (as far as I 
know), this might not be desirable for you. As a final point, I'd say 
that although it might not be ISO, it's *extremely* useful!

Kind regards,
Matt Lilley

[1] http://www.swi-prolog.org/pldoc/doc_for?object=setup_call_cleanup/3

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Matt Lilley
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