Hi Doug,
> Hmmm... the clauses seem paired in like so, in the or/2 reference, like
>
> (or
> ((equal 3 @X) (equal @X 4))
> ((equal 7 @X) (equal @X 7)) )
>
>
> When I similarly pair the clauses of or/2:
>
> (be foo (@N) (or ((a @N) (b @N))))
Right. This is how Pilog (Prolog) works. ;-)
The most fundamental rule is that all clauses in a rule are combined by
an implicit AND:
# 'b' AND 'c' must be true for 'a' to succeed
(be a (@X @Y)
(b @X)
(c @Y) )
To get an OR, you write several rules:
# ('b' AND 'c') OR ('d' AND 'e') must be true for 'a' to succeed
(be a (@X @Y)
(b @X)
(c @Y) )
(be a (@X @Y)
(d @X)
(e @Y) )
Therefore, the 'or' predicate is not really necessary. It is just a
convenience, to avoid defining multiple rules. But from the above it
follows that the each argument clause to 'or' must itself be a list of
clauses:
# ('b' AND 'c') OR ('d' AND 'e') must be true for 'a' to succeed
(be a (@X @Y)
(or
((b @X) (c @Y))
((d @X) (e @Y)) ) )
(From your next mail):
> Alex, would it be possible to give pilog an and/2 rule similar to the
> or/2 rule that pilog has now?
Now it should be clear that an 'and/2' predicate is superfluous, because
AND is implicit in all Prolog rules.
Cheers,
- Alex
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