Module:Name/Arity
(or‘//Arity for non-terminals), both the module and
arity may be omitted in which case Pred refers to all
matching predicates. PortSpec is either a single port (call, exit, fail
or redo), preceded with + or - or
a list of these. The predicate modifies the current trace specification
and then installs a suitable wrapper for the predicate using wrap_predicate/4.
For example:
?- trace(append). % lists:append/2: [all] % lists:append/3: [all] % append/1: [all] true. ?- append([a,b], [c], L). T [10] Call: lists:append([a, b], [c], _18032) T [19] Call: lists:append([b], [c], _19410) T [28] Call: lists:append([], [c], _20400) T [28 +0.1ms] Exit: lists:append([], [c], [c]) T [19 +0.2ms] Exit: lists:append([b], [c], [b, c]) T [10 +0.5ms] Exit: lists:append([a, b], [c], [a, b, c]) L = [a, b, c]. ?- trace(append, -all). % lists:append/2: Not tracing % lists:append/3: Not tracing % append/1: Not tracing
The text between [] indicates the call depth (first
number) and for all ports except the call port the wall
time since the start (call port) in milliseconds. Note that the
instrumentation and print time is included in the time. In the example
above the actual time is about 0.00001ms on todays hardware.
In addition, conditions may be specified. In this case the the
specification takes the shape trace(:Head, Port(Condition)).
For example:
?- trace(current_prolog_flag(Flag, Value), call(var(Flag))). ?- list_tracing. % Trace points (see trace/1,2) on: % system:current_prolog_flag(A,_): [call(var(A))]
This specification will only print the goal if the registered condition succeeds. Note that we can use the condition for its side effect and then fail to avoid printing the event. Clearing the trace event on all relevant ports removes the condition. There is currently no way to modify the condition without clearing the trace point first.
?- scanl(plus, [1,2,3,4,5], 0, Sums). Sums = [0, 1, 3, 6, 10, 15].
When considering the lists columns of a table, scanl/5 combines the accumulator with each row to produce the next value in the result list. We illustrate this using plus/4 which adds the accumulator with the values from the row to produce the next accumulator. We use trace/2 to illustrate how the values are combined.
plus(A,B,C,Result) :- Result is A+B+C.
?- trace(plus/4, exit). % plus/4: [exit] ?- scanl(plus, [1,2,3,4,5], [10,20,30,40,50], 0, Sums). T Exit: plus(1, 10, 0, 11) T Exit: plus(2, 20, 11, 33) T Exit: plus(3, 30, 33, 66) T Exit: plus(4, 40, 66, 110) T Exit: plus(5, 50, 110, 165) Sums = [0, 11, 33, 66, 110, 165].