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prolog_stack.pl -- Examine the Prolog stack
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This module defines high-level primitives for examining the Prolog stack, primarily intended to support debugging. It provides the following functionality:

This library may be enabled by default to improve interactive debugging, for example by adding the lines below to your <config>/init.pl to decorate uncaught exceptions:

:- use_module(library(prolog_stack)).
- Use of this library may negatively impact performance of applications that process (error-)exceptions frequently as part of their normal processing.
Source get_prolog_backtrace(+MaxDepth, -Backtrace) is det
Source get_prolog_backtrace(+MaxDepth, -Backtrace, +Options) is det
Obtain a backtrace from the current location. The backtrace is a list of frames. Each frame is an opaque term that can be inspected using the predicate prolog_stack_frame_property/2 can be used to extract information from these frames. Most use scenarios will pass the stack to print_prolog_backtrace/2. The following options are provided:
Start at Frame instead of the current frame.
If Depth > 0, include a shallow copy of the goal arguments into the stack. Default is set by the Prolog flag backtrace_goal_depth, set to 3 initially, showing the goal and toplevel of any argument.
Do not show stack frames above Guard. See stack_guard/1.
Report locations as Clause+PC or as a location term that does not use clause references, allowing the exception to be printed safely in a different context.
Frame- is the frame to start from. See prolog_current_frame/1.
MaxDepth- defines the maximum number of frames returned.
- get_prolog_backtrace/3 used to have the parameters +Frame, +MaxDepth, -Backtrace. A call that matches this signature is mapped to get_prolog_backtrace(MaxDepth, Backtrace, [frame(Frame)]).
Source clause_where(+UseClauseRef, +Clause, +PC, -Where, +Options) is det[private]
Get a description of the frame source location from Clause and the PC inside clause. If UseClauseRef is true, this is the a term clause(Clause,PC), providing all abvailable information to the caller at low time overhead. If however the exception need to be printed in an environment where the clause references may differ, for example because the program is not loaded, it is printed in a different thread and contains references to dynamic predicates, etc, it is better to use the information inside the clause here.
Source copy_goal(+TermDepth, +Frame, -Goal) is det[private]
Create a shallow copy of the frame's goal to help debugging. In addition to shallow copying, high-arity terms are represented as below. Currently the 16 first arguments are hardcoded.
name(A1, ..., A16, <skipped Skipped of Arity>, An)
Source prolog_stack_frame_property(+Frame, ?Property) is nondet
True when Property is a property of Frame. Frame is an element of a stack-trace as produced by get_prolog_backtrace/2. Defined properties are:
  • level(Level)
  • predicate(PI)
  • location(File:Line)
Source print_prolog_backtrace(+Stream, +Backtrace) is det
Source print_prolog_backtrace(+Stream, +Backtrace, +Options) is det
Print a stacktrace in human readable form to Stream. Options is an option list that accepts:
If true, print subgoal line numbers. The default depends on the Prolog flag backtrace_show_lines.
Backtrace- is a list of frame(Depth,Location,Goal) terms.
Source clause_predicate_name(+ClauseRef, -Predname) is det[private]
Produce a name (typically Functor/Arity) for a predicate to which Clause belongs.
Source backtrace(+MaxDepth)
Get and print a stacktrace to the user_error stream.
Source find_subgoal(+PosList, +TermPos, -SubGoalPos)[private]
See also
- Same as find_subgoal/3 in trace.pl from the GUI tracer.
Source lineno(+File, +Char, -Line)[private]
Translate a character location to a line-number. Note that this calls try_open_source/2, but this is always loaded as we would not have clause info without.
Source print_last_choicepoint is det
Print details on the last open choice point.
Source print_last_choicepoint(+ChoiceRef, +Options) is det
Source prolog_stack:stack_guard(+PI) is semidet[multifile]
Dynamic multifile hook that is normally not defined. The hook is called with PI equal to none if the exception is not caught and with a fully qualified (e.g., Module:Name/Arity) predicate indicator of the predicate that called catch/3 if the exception is caught.

The exception is of the form error(Formal, ImplDef) and this hook succeeds, ImplDef is unified to a term context(prolog_stack(StackData), Message). This context information is used by the message printing system to print a human readable representation of the stack when the exception was raised.

For example, using a clause stack_guard(none) prints contexts for uncaught exceptions only. Using a clause stack_guard(_) prints a full stack-trace for any error exception if the exception is given to print_message/2. See also library(http/http_error), which limits printing of exceptions to exceptions in user-code called from the HTTP server library.

Details of the exception decoration is controlled by two Prolog flags:

Integer that controls the maximum number of frames collected. Default is 20. If a guard is specified, callers of the guard are removed from the stack-trace.
Boolean that indicates whether the library tries to find line numbers for the calls. Default is true.
Source stack_guard(+Reason) is semidet[multifile]
Dynamic multifile predicate. It is called with none, 'C' or the predicate indicator of the guard, the predicate calling catch/3. The exception must be of compatible with the shape error(Formal, context(Stack, Msg)). The default is to catch none, uncaught exceptions. 'C' implies that the callback from C will handle the exception.