🚀 go-pugleaf

In article <MATSC.92Sep2141625@vishnu.sics.se>, matsc@sics.se (Mats Carlsson) writes:
>    do you do anything special about this construct, or do you just
>    consider it as an ordinary predicate with a definition like:
>    P ;_Q :- P.
>    _P; Q :- Q.
> 
> SICStus Prolog does nothing special, and considers it as an ordinary
> predicate as you describe.

That cannot be so, because SICStus Prolog gets the right answer for this
program:

    p :- ( q(1), ! ; q(2) ).
    p :- q(3).

    q(X) :- write(X), nl.

    ?- p, fail.

The right answer is to write 1 and then fail.  If it were done this way:

    p :- 'OR'( (q(1),!), q(2) ).
    p :- q(3).

    'OR'(P, Q) :- call(P).
    'OR'(P, Q) :- call(Q).

    q(X) :- write(X), nl.

which _would_ treat ; as an ordinary predicate, then the output would
be 1 2 3.  SICStus Prolog gets this right, so there has to be _something_
special about disjunction.

-- 
You can lie with statistics ... but not to a statistician.