🚀 go-pugleaf

In article 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.

I guess this is slightly understated, because that would mean that all goals
inside a disjunction are metacalled, which is not the case in SICStus.

There are two basic ways to compile the disjunctive constructs, one is with
auxiliary procedures and the other in-line. If you have a clause

	p(X, Y) :- q, (r(X); s(Y)), t.

then the former method (used e.g. in SICStus, KCM, ...) is to transform it into

	p(X, Y) :- q, aux(X, Y), t.

	aux(X, _) :- r(X).
	aux(_, Y) :- s(Y).

(in case there is a cut involved, one might need to pass additional information
to aux/2).

The other method is done at the WAM level, the WAM code looks something like

	allocate
	get_variable Y1, A1
	get_variable Y2, A2
	call q/0
	try_me_else L1
	put_value Y1, A1
	call r/1
	branch L2
L1:	trust_me
	put_value Y2, A1
	call s/1
L2:	deallocate
	execute t/0

This method is used e.g. by (as far as I know) Quintus and Sepia and others.
Both methods have advantages and disadvantages. I think the latter is more
efficient, because no additional environment is created for the auxiliary
procedures and the disjunction choice point has always arity zero. There might as well
be less branches to make. On the other hand, the former method can be easily
done on the Prolog level with a preprocessor, so it is quite easy (well, modulo
cut and the like) to do. The in-line expansion, especially if one wants to
improve special cases like not/1, once/1 and if-then-else with a simple condition,
can very quickly turn into a dangerous adventure.

--Micha

---
Micha Meier			------------------------------------------------
ECRC, Arabellastr. 17		The opinions expressed above are private
D-8000 Munich 81		and may not reflect those of my employer.
micha@ecrc.de, Tel. +49-89-92699-108, Fax  +49-89-92699-170