🚀 go-pugleaf

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):-
 	  ...

The code of this canonical form is efficient because (1) no choice point will
be created, and (2) it does not contain instructions for the cuts.

Neng-Fa Zhou 
Kyushu University