🚀 go-pugleaf

In article <ZHOU.90Nov20145919@hiromi.csce.kyushu-u.junet> zhou@hiromi.csce.kyushu-u.junet (Neng Fa Zhou) writes:

   In article <DOWDING.90Nov16140150@palm.ai.sri.com> dowding@ai.sri.com (John Dowding) writes:

   > I was poking around in some old code, and came across the following
   > clauses (names changed to protect the inocent):
   > 
   > foo(Term, Term):-
   > 	  (var(Term) ; atomic(Term)),
   > 	  !.
   > foo([Head|Tail], Result):-
   > 	  ...
   > 
   > 
   > I was wondering if the following is more efficient:
   > 
   > foo(Term, Term):-
   > 	  var(Term),
   > 	  !.
   > foo(Term, Term):-
   > 	  atomic(Term),
   > 	  !.
   > foo([Head|Tail], Result):-
   > 	  ...

   Yes, it is more efficient than the original one.

   > 
   > Are Prolog compilers (particularly, Quintus version 2.4) smart enough 
   > index correctly on the 1st argument in either or both of these cases?
   >

    The compiler for the TOAM is smart enough to compile this program to 
   efficient code. The compiler first infers modes for a program, then 
   transforms the program to a canonical form based on the inferred modes.
   Suppose [d,f] is the mode of foo/2, the canonical form of the predicate is

    foo(Term, NewTerm):-
	     var(Term),
	     !,
	     NewTerm = Term.
    foo(Term, NewTerm):-
	     atomic(Term),
	     !,
	     NewTerm = Term.
    foo([Head|Tail], Result):-
	     ...

This canonical form was probably what the original program should have
looked like anyway. The intension was probably that the first
clause should have been the only that should have be applicable if the
first argument is a variable. However, the call :-foo(X,frobozz)
matches the last clause, which is probably wrong. 

It is a very common error to write the clause

bar(X,X) :-
	foo(X),
	!.

instead of the clause

bar(X,Y) :-
	foo(X),
	!,
	X=Y.


The last program is more efficient, since the unification X=Y
is only done if the predicate foo(X) before the cut succeeds. If foo
is a predicate like atomic, var... the indexing of SICStus will make
the program above very efficient. (I also believe that SICStus will
expand the disjunction into two clauses.) 

--
Mattias Waldau                              
Computing Science Department                mattias@emil.csd.uu.se
P.O. Box 520, S-751 20  Uppsala, Sweden     Phone: +46-18-181055