Tabled Predicates and Tracing

Another issue to be aware of when using tabling in XSB is tracing. XSB's tracer is a standard 4-port tracer that interacts with the engine at each call, exit, redo, and failure of a predicate (see Chapter 8). When tabled predicates are traced, these events may occur in unexpected ways, as the following example shows.

Exercise 5.2.8  

Consider a tabled evaluation when the query ?- a(0,X) is given to the following program

:- table mut_ret_a/2, mut_ret_b/2.
mut_ret_a(X,Y) :- mut_ret_d(X,Y).
mut_ret_a(X,Y) :- mut_ret_b(X,Z),mut_ret_c(Z,Y).

mut_ret_b(X,Y) :- mut_ret_c(X,Y).
mut_ret_b(X,Y) :- mut_ret_a(X,Z),mut_ret_d(Z,Y).

mut_ret_c(2,2).      mut_ret_c(3,3).

mut_ret_d(0,1).	     mut_ret_d(1,2).     mut_ret_d(2,3).

mut_ret_a(0,1) can be derived immediately from the first clause of mut_ret_a/2. All other answers to the query depend on answers to the subgoal mut_ret_b(0,X) which arises in the evaluation of the second clause of mut_ret_a/2. Each answer to mut_ret_b(0,X) in turn depends on an answer to mut_ret_a(0,X), so that the evaluation switches back and forth between deriving answers for mut_ret_a(0,X) and mut_ret_b(0,X).

Try tracing this evaluation, using creep and skip. Do you find the behavior intuitive or not?$\Box$