Print passes through ports of specified predicates. Pred
is a, possible partial, specification of a predicate as it is also used
be
spy/1 and similar
predicates. Where a full predicate specification is of the shape 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.
