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prolog_trace.pl -- Print access to predicates
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This library prints accesses to specified predicates by wrapping the predicate.

See also
- library(debug) for adding conditional print statements to a program.
Source trace(:Pred) is det
Source trace(:Pred, +PortSpec) is det
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]

?- 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.

Source tracing(:Spec, -Ports)
True if Spec is traced using Ports
Source list_tracing
List predicates we are currently tracing
Source notraceall is det
Remove all trace points

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

Source trace(Arg1, Arg2)