prolog_stack.pl -- Examine the Prolog stack

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

• get_prolog_backtrace/2 gets a Prolog representation of the Prolog stack. This can be used for printing, but also to enrich exceptions using prolog_exception_hook/4. Decorating exceptions is provided by this library and controlled by the hook stack_guard/1.
• print_prolog_backtrace/2 prints a backtrace as returned by get_prolog_backtrace/2
• The shorthand backtrace/1 fetches and prints a backtrace.

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)).

bug

- Use of this library may negatively impact performance of applications that process (error-)exceptions frequently as part of their normal processing.

 get_prolog_backtrace(+MaxDepth, -Backtrace) is det

 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:

frame(+Frame)

Start at Frame instead of the current frame.

goal_term_depth(+Depth)

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.

guard(+Guard)

Do not show stack frames above Guard. See stack_guard/1.

clause_references(+Bool)

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.

Arguments:

Frame	- is the frame to start from. See prolog_current_frame/1.
MaxDepth	- defines the maximum number of frames returned.

Compatibility

- 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)]).

 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.

 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)

 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)

 print_prolog_backtrace(+Stream, +Backtrace) is det

 print_prolog_backtrace(+Stream, +Backtrace, +Options) is det

Print a stacktrace in human readable form to Stream. Options is an option list that accepts:

subgoal_positions(+Boolean)

If true, print subgoal line numbers. The default depends on the Prolog flag backtrace_show_lines.

Arguments:

Backtrace

- is a list of frame(Depth,Location,Goal) terms.

 clause_predicate_name(+ClauseRef, -Predname) is det[private]

Produce a name (typically Functor/Arity) for a predicate to which Clause belongs.

 backtrace(+MaxDepth)

Get and print a stacktrace to the user_error stream.

 find_subgoal(+PosList, +TermPos, -SubGoalPos)[private]

See also

- Same as find_subgoal/3 in trace.pl from the GUI tracer.

 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.

 print_last_choicepoint is det

Print details on the last open choice point.

 print_last_choicepoint(+ChoiceRef, +Options) is det

 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:

backtrace_depth

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.

backtrace_show_lines

Boolean that indicates whether the library tries to find line numbers for the calls. Default is true.

 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.

Undocumented predicates

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

 get_prolog_backtrace(Arg1, Arg2, Arg3)

 print_prolog_backtrace(Arg1, Arg2, Arg3)