Prolog: redundant results in sentences related to anonymous variables

Question

Consider the following Prolog program.

a(X) :- b(_), c(X).

b(1).
b(2).
b(3).

c(1).

Request run:

a(X).

in SWI-Prolog, we get three results: all X = 1.

Given that we do not care about the anonymous variable, what prevents SWI-Prolog from returning a single result? Why is this optimization not performed?

thank

Answer 1

Good for Prolog, underscores are just an anonymous variable. Therefore, the predicate is a/1equivalent to:

a(X) :-
    b(Y),
    c(X).

b(Y), Y , , . , Y X, b(Y) X.

Prolog, , , :

• b/1 /. , :

  b(a) :-
      print(a).
  b(a) :-
      print(b).
  

  a b .

• b/1 , ,... . , , ,

• b/1 asserta/1, assertz/1 .. . , , c/1, c/1 .

• Prolog , , .

• , b/1 c/1.

b/1 once/1 -. :

a(X) :-
    once(b(_)),
    c(X).