Re: get unused integer

On 2008-05-19, Peter Van Weert <Peter.VanWeert@xxxxxxxxxxxxxx> wrote:
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Simon Strobl schreef:
As my program will use multiple threads anyway (although only at a
later stage of development), it probably makes sense to adjust the
label-assigning technique to this fact.

I think I can assign an identity to each parsing procedure (s(T1, T3,
Procedure) ...) and use the following simple code:

:- dynamic(assigned/2).

get_new_label(Label, Procedure) :-
label(Label),
not(assigned(Label, Procedure)),
assert(assigned(Label, Procedure)).
...
I never used multiple threads in (Swi) Prolog before.

So, maybe someone with greater experience would like to comment on
this code? Is it safe? Is it efficient?

I never wrote multithreaded Prolog code either, but even then it is
obvious to me that

not(assigned(Label, Procedure)),
assert(assigned(Label, Procedure))

cannot be threadsafe. You'll need some form of synchronisation here.
Section 8.4 of the SWI manual seems a good place to start
(http://gollem.science.uva.nl/SWI-Prolog/Manual/threadsync.html).

Yip. Also a non-deterministic label/1 is most likely not what you
want. Finally, you'll start trying labels from 0 each time, so this
will get pretty slow eventually. Typically I'd use something like
this:

:- dynamic
last_label/1,
assigned/2.

get_new_label(Label, Procedure) :-
with_mutex(label, assign_label(Label, Procedure)).

assign_label(Label, Procedure) :-
retract(last_label(Num)), !,
Label is Num+1,
assert(last_label(Label)),
assert(assigned(Label, Procedure)).
assign_label(Label, Procedure) :-
assert(last_label(1)),
assert(assigned(1, Procedure)).

--- Jan
.

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