Tracer and debugger

SWI-Prolog tracer and debugger

SWI-Prolog offers a native text-console based tracer and debugger.

Text-console based tracer

You can trace Prolog's computations by giving the command ?-trace. This will put Prolog in trace mode, showing every single resolution step (except for the predicates that have been compiled rather than consulted). Trace mode is switched off by the command ?-notrace.

If you only want to debug small part of the code you can set spy and trace points and use ?-debug. command (and ?-nodebug. to disable debugging).

For more help see SWI-Prolog API available here.

Tracing

We will follow the computation trace of the query ?-student_of(S,peter)., depicted in Figure 3.1 on page 45 of the book. Load the file student_of.pl.

?- trace.

Yes
[trace]  ?- student_of(S,peter).
   Call: (7) student_of(_G311, peter) ? creep
   Call: (8) follows(_G311, _L210) ? creep
   Exit: (8) follows(paul, computer_science) ? creep
   Call: (8) teaches(peter, computer_science) ? creep
   Exit: (8) teaches(peter, computer_science) ? creep
   Exit: (7) student_of(paul, peter) ? creep

S = paul ;

After each line of output we can give a command; h lists the possible options.

+:                  spy        -:              no spy
/c|e|r|f|u|a goal:  find       .:              repeat find
a:                  abort      A:              alternatives
b:                  break      c (ret, space): creep
[depth] d:          depth      e:              exit
f:                  fail       [ndepth] g:     goals (backtrace)
h (?):              help       i:              ignore
l:                  leap       L:              listing
n:                  no debug   p:              print
r:                  retry      s:              skip
u:                  up         w:              write
m:                    exception details
C:                  toggle show context

For the moment we are just stepping (creeping) through the computation by hitting RETURN. The relation with SLD-trees is as follows. A Call means passing through a node in the SLD-tree in downward direction; only the first literal of the resolvent is shown. Exit means passing upward through a node. The number to the left indicates the depth of the node in the SLD-tree, but the SWI-Prolog pl shell starts counting at 7 instead of 1 (why SWI does this is left as an exercise for the reader). So, for instance, teaches(peter,computer_science) is called at level 8 (i.e. level 2 in the SLD-tree).

We have found our first solution, and force backtracking by typing a semi-colon as usual. We thus backtrack to the most recent choice point.

   Redo: (8) follows(_G311, _L210) ? creep
   Exit: (8) follows(paul, expert_systems) ? creep
   Call: (8) teaches(peter, expert_systems) ? creep
   Fail: (8) teaches(peter, expert_systems) ? creep
   Redo: (8) follows(_G311, _L210) ? creep
   Exit: (8) follows(maria, ai_techniques) ? creep
   Call: (8) teaches(peter, ai_techniques) ? creep
   Exit: (8) teaches(peter, ai_techniques) ? creep
   Exit: (7) student_of(maria, peter) ? creep

S = maria ;

Redo indicates the search for an alternative solution; notice that the literal following Redo is the most recently found answer rather than the query to which we seek an alternative solution (see the Call at the same level). The second solution for follows(S,C) leads to a failure branch, because we can't solve teaches(peter,expert_systems). We thus redo, after which we find our second solution. teaches(peter,ai_techniques) is not the last teaches fact in the program. Forced backtracking however shows that all solutions have been exhausted.

   Redo: (8) teaches(peter, ai_techniques) ? creep
   Fail: (7) student_of(_G311, peter) ? creep

No