Re: Aborting without killing

Thanks Jan,

Ah, ok I get it now. The reason I want to stop the execution of a goal
(any goal) without killing the thread is that the dynamic db contains
clauses declared as thread_local, and I don't want them to disappear.

Best,
Torbjörn

On Thu, Nov 14, 2013 at 3:44 PM, Jan Wielemaker <[email protected]> wrote:
> On 11/14/2013 01:44 PM, Torbjörn Lager wrote:
>> Hi,
>>
>> Running (say)
>>
>> ?- repeat, fail.
>>
>> and executing thread_signal(main, abort) from another thread results in
>>
>> % Execution Aborted
>>
>> However, main is still running.
>>
>> However, running
>>
>> ?- thread_create((repeat, fail), ID, [alias(mymain)]).
>>
>> and executing thread_signal(mymain, abort) from another thread results
>> in mymain terminating with an exception '$aborted'.
>>
>> So, is there a way to abort a currently running goal without also
>> killing the thread that runs it? Or is the thread 'main' in the above
>> example killed and then recreated again? Is that what is happening?
>
> The main thread runs the C-function PL_toplevel(), which restarts the
> engine if it died on an exception.  This is the same for other exceptions:
> an exception does not kill the interactive toplevel, but does kill any
> other thread when not caught.
>
> What is special about abort, which is implemented as throw('$aborted'),
> is that catch wraps the recover goal to re-throw the exception after
> the recover goal has terminated.  So, you can cheat and make sure the
> recover goal never terminates :-)
>
> The big question of course is why you'd like to use abort if you don't
> want to kill the thread.  If you just want it stop the thread doing some
> task and start another, simply do signal(Target, throw(stop)) and in the
> target, you do
>
>         catch(Goal, stop, <whatever>)
>
>         Cheers --- Jan



-- 
Torbjörn Lager
Professor of General and Computational Linguistics
Department of Philosophy, Linguistics and Theory of Science
University of Gothenburg
Box 200, SE-405 30 Gothenburg, Sweden
Phone: +46317864962