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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2004-2021, University of Amsterdam
    7                              VU University Amsterdam
    8                              SWI-Prolog Solutions b.v.
    9    All rights reserved.
   10
   11    Redistribution and use in source and binary forms, with or without
   12    modification, are permitted provided that the following conditions
   13    are met:
   14
   15    1. Redistributions of source code must retain the above copyright
   16       notice, this list of conditions and the following disclaimer.
   17
   18    2. Redistributions in binary form must reproduce the above copyright
   19       notice, this list of conditions and the following disclaimer in
   20       the documentation and/or other materials provided with the
   21       distribution.
   22
   23    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   24    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   25    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   26    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   27    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   28    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   29    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   30    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   31    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   32    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   33    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   34    POSSIBILITY OF SUCH DAMAGE.
   35*/
   36
   37:- module(prolog_stack,
   38          [ get_prolog_backtrace/2,     % +MaxDepth, -Stack
   39            get_prolog_backtrace/3,     % +MaxDepth, -Stack, +Options
   40            prolog_stack_frame_property/2, % +Frame, ?Property
   41            print_prolog_backtrace/2,   % +Stream, +Stack
   42            print_prolog_backtrace/3,   % +Stream, +Stack, +Options
   43            backtrace/1,                % +MaxDepth
   44            print_last_choicepoint/0,
   45            print_last_choicepoint/2    % +Choice, +Options
   46          ]).   47:- autoload(library(debug),[debug/3]).   48:- autoload(library(error),[must_be/2]).   49:- autoload(library(lists),[nth1/3,append/3]).   50:- autoload(library(option),[option/2,option/3,merge_options/3]).   51:- autoload(library(prolog_clause),
   52	    [clause_name/2,predicate_name/2,clause_info/4]).   53
   54
   55:- dynamic stack_guard/1.   56:- multifile stack_guard/1.   57
   58:- predicate_options(print_prolog_backtrace/3, 3,
   59                     [ subgoal_positions(boolean)
   60                     ]).

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:

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. */
   92:- create_prolog_flag(backtrace,            true, [type(boolean), keep(true)]).   93:- create_prolog_flag(backtrace_depth,      20,   [type(integer), keep(true)]).   94:- create_prolog_flag(backtrace_goal_depth, 3,    [type(integer), keep(true)]).   95:- create_prolog_flag(backtrace_show_lines, true, [type(boolean), keep(true)]).
 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)]).
  128get_prolog_backtrace(MaxDepth, Stack) :-
  129    get_prolog_backtrace(MaxDepth, Stack, []).
  130
  131get_prolog_backtrace(Fr, MaxDepth, Stack) :-
  132    integer(Fr), integer(MaxDepth), var(Stack),
  133    !,
  134    get_prolog_backtrace_lc(MaxDepth, Stack, [frame(Fr)]),
  135    nlc.
  136get_prolog_backtrace(MaxDepth, Stack, Options) :-
  137    get_prolog_backtrace_lc(MaxDepth, Stack, Options),
  138    nlc.            % avoid last-call-optimization, such that
  139                        % the top of the stack is always a nice Prolog
  140                        % frame
  141
  142nlc.
  143
  144get_prolog_backtrace_lc(MaxDepth, Stack, Options) :-
  145    (   option(frame(Fr), Options)
  146    ->  PC = call
  147    ;   prolog_current_frame(Fr0),
  148        prolog_frame_attribute(Fr0, pc, PC),
  149        prolog_frame_attribute(Fr0, parent, Fr)
  150    ),
  151    (   option(goal_term_depth(GoalDepth), Options)
  152    ->  true
  153    ;   current_prolog_flag(backtrace_goal_depth, GoalDepth)
  154    ),
  155    option(guard(Guard), Options, none),
  156    (   def_no_clause_refs(Guard)
  157    ->  DefClauseRefs = false
  158    ;   DefClauseRefs = true
  159    ),
  160    option(clause_references(ClauseRefs), Options, DefClauseRefs),
  161    must_be(nonneg, GoalDepth),
  162    backtrace(MaxDepth, Fr, PC, GoalDepth, Guard, ClauseRefs, Stack, Options).
  163
  164def_no_clause_refs(system:catch_with_backtrace/3).
  165
  166backtrace(0, _, _, _, _, _, [], _) :- !.
  167backtrace(MaxDepth, Fr, PC, GoalDepth, Guard, ClauseRefs,
  168          [frame(Level, Where, Goal)|Stack], Options) :-
  169    prolog_frame_attribute(Fr, level, Level),
  170    (   PC == foreign
  171    ->  prolog_frame_attribute(Fr, predicate_indicator, Pred),
  172        Where = foreign(Pred)
  173    ;   PC == call
  174    ->  prolog_frame_attribute(Fr, predicate_indicator, Pred),
  175        Where = call(Pred)
  176    ;   prolog_frame_attribute(Fr, clause, Clause)
  177    ->  clause_where(ClauseRefs, Clause, PC, Where, Options)
  178    ;   Where = meta_call
  179    ),
  180    (   Where == meta_call
  181    ->  Goal = 0
  182    ;   copy_goal(GoalDepth, Fr, Goal)
  183    ),
  184    (   prolog_frame_attribute(Fr, pc, PC2)
  185    ->  true
  186    ;   PC2 = foreign
  187    ),
  188    (   prolog_frame_attribute(Fr, parent, Parent),
  189        prolog_frame_attribute(Parent, predicate_indicator, PI),
  190        PI == Guard                             % last frame
  191    ->  backtrace(1, Parent, PC2, GoalDepth, Guard, ClauseRefs, Stack, Options)
  192    ;   prolog_frame_attribute(Fr, parent, Parent),
  193        more_stack(Parent)
  194    ->  D2 is MaxDepth - 1,
  195        backtrace(D2, Parent, PC2, GoalDepth, Guard, ClauseRefs, Stack, Options)
  196    ;   Stack = []
  197    ).
  198
  199more_stack(Parent) :-
  200    prolog_frame_attribute(Parent, predicate_indicator, PI),
  201    \+ (   PI = ('$toplevel':G),
  202           G \== (toplevel_call/1)
  203       ),
  204    !.
  205more_stack(_) :-
  206    current_prolog_flag(break_level, Break),
  207    Break >= 1.
 clause_where(+UseClauseRef, +Clause, +PC, -Where, +Options) is det
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.
  220clause_where(true, Clause, PC, clause(Clause, PC), _).
  221clause_where(false, Clause, PC, pred_line(PredName, File:Line), Options) :-
  222    option(subgoal_positions(true), Options, true),
  223    subgoal_position(Clause, PC, File, CharA, _CharZ),
  224    File \= @(_),                 % XPCE Object reference
  225    lineno(File, CharA, Line),
  226    clause_predicate_name(Clause, PredName),
  227    !.
  228clause_where(false, Clause, _PC, pred_line(PredName, File:Line), _Options) :-
  229    clause_property(Clause, file(File)),
  230    clause_property(Clause, line_count(Line)),
  231    clause_predicate_name(Clause, PredName),
  232    !.
  233clause_where(false, Clause, _PC, clause_name(ClauseName), _Options) :-
  234    clause_name(Clause, ClauseName).
 copy_goal(+TermDepth, +Frame, -Goal) is det
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)
  246copy_goal(0, _, 0) :- !.                        % 0 is not a valid goal
  247copy_goal(D, Fr, Goal) :-
  248    prolog_frame_attribute(Fr, goal, Goal0),
  249    (   Goal0 = Module:Goal1
  250    ->  copy_term_limit(D, Goal1, Goal2),
  251        (   hidden_module(Module)
  252        ->  Goal = Goal2
  253        ;   Goal = Module:Goal2
  254        )
  255    ;   copy_term_limit(D, Goal0, Goal)
  256    ).
  257
  258hidden_module(system).
  259hidden_module(user).
  260
  261copy_term_limit(0, In, '...') :-
  262    compound(In),
  263    !.
  264copy_term_limit(N, In, Out) :-
  265    is_dict(In),
  266    !,
  267    dict_pairs(In, Tag, PairsIn),
  268    N2 is N - 1,
  269    MaxArity = 16,
  270    copy_pairs(PairsIn, N2, MaxArity, PairsOut),
  271    dict_pairs(Out, Tag, PairsOut).
  272copy_term_limit(N, In, Out) :-
  273    compound(In),
  274    !,
  275    compound_name_arity(In, Functor, Arity),
  276    N2 is N - 1,
  277    MaxArity = 16,
  278    (   Arity =< MaxArity
  279    ->  compound_name_arity(Out, Functor, Arity),
  280        copy_term_args(0, Arity, N2, In, Out)
  281    ;   OutArity is MaxArity+2,
  282        compound_name_arity(Out, Functor, OutArity),
  283        copy_term_args(0, MaxArity, N2, In, Out),
  284        SkipArg is MaxArity+1,
  285        Skipped is Arity - MaxArity - 1,
  286        format(atom(Msg), '<skipped ~D of ~D>', [Skipped, Arity]),
  287        arg(SkipArg, Out, Msg),
  288        arg(Arity, In, InA),
  289        arg(OutArity, Out, OutA),
  290        copy_term_limit(N2, InA, OutA)
  291    ).
  292copy_term_limit(_, In, Out) :-
  293    copy_term_nat(In, Out).
  294
  295copy_term_args(I, Arity, Depth, In, Out) :-
  296    I < Arity,
  297    !,
  298    I2 is I + 1,
  299    arg(I2, In, InA),
  300    arg(I2, Out, OutA),
  301    copy_term_limit(Depth, InA, OutA),
  302    copy_term_args(I2, Arity, Depth, In, Out).
  303copy_term_args(_, _, _, _, _).
  304
  305copy_pairs([], _, _, []) :- !.
  306copy_pairs(Pairs, _, 0, ['<skipped>'-Skipped]) :-
  307    !,
  308    length(Pairs, Skipped).
  309copy_pairs([K-V0|T0], N, MaxArity, [K-V|T]) :-
  310    copy_term_limit(N, V0, V),
  311    MaxArity1 is MaxArity - 1,
  312    copy_pairs(T0, N, MaxArity1, T).
 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:
  325prolog_stack_frame_property(frame(Level,_,_), level(Level)).
  326prolog_stack_frame_property(frame(_,Where,_), predicate(PI)) :-
  327    frame_predicate(Where, PI).
  328prolog_stack_frame_property(frame(_,clause(Clause,PC),_), location(File:Line)) :-
  329    subgoal_position(Clause, PC, File, CharA, _CharZ),
  330    File \= @(_),                   % XPCE Object reference
  331    lineno(File, CharA, Line).
  332prolog_stack_frame_property(frame(_,_,_,Goal), goal(Goal)) :-
  333    Goal \== 0.
  334
  335
  336frame_predicate(foreign(PI), PI).
  337frame_predicate(call(PI), PI).
  338frame_predicate(clause(Clause, _PC), PI) :-
  339    clause_property(Clause, PI).
  340
  341default_backtrace_options(Options) :-
  342    (   current_prolog_flag(backtrace_show_lines, true)
  343    ->  Options = []
  344    ;   Options = [subgoal_positions(false)]
  345    ).
 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.
  359print_prolog_backtrace(Stream, Backtrace) :-
  360    print_prolog_backtrace(Stream, Backtrace, []).
  361
  362print_prolog_backtrace(Stream, Backtrace, Options) :-
  363    default_backtrace_options(DefOptions),
  364    merge_options(Options, DefOptions, FinalOptions),
  365    phrase(message(Backtrace, FinalOptions), Lines),
  366    print_message_lines(Stream, '', Lines).
  367
  368:- public                               % Called from some handlers
  369    message//1.  370
  371message(Backtrace) -->
  372    {default_backtrace_options(Options)},
  373    message(Backtrace, Options).
  374
  375message(Backtrace, Options) -->
  376    message_frames(Backtrace, Options),
  377    warn_nodebug(Backtrace).
  378
  379message_frames([], _) -->
  380    [].
  381message_frames([H|T], Options) -->
  382    message_frames(H, Options),
  383    (   {T == []}
  384    ->  []
  385    ;   [nl],
  386        message_frames(T, Options)
  387    ).
  388
  389message_frames(frame(Level, Where, 0), Options) -->
  390    !,
  391    level(Level),
  392    where_no_goal(Where, Options).
  393message_frames(frame(Level, _Where, '$toplevel':toplevel_call(_)), _) -->
  394    !,
  395    level(Level),
  396    [ '<user>'-[] ].
  397message_frames(frame(Level, Where, Goal), Options) -->
  398    level(Level),
  399    [ ansi(code, '~p', [Goal]) ],
  400    where_goal(Where, Options).
  401
  402where_no_goal(foreign(PI), _) -->
  403    [ '~w <foreign>'-[PI] ].
  404where_no_goal(call(PI), _) -->
  405    [ '~w'-[PI] ].
  406where_no_goal(pred_line(PredName, File:Line), _) -->
  407    !,
  408    [ '~w at '-[PredName], url(File:Line) ].
  409where_no_goal(clause_name(ClauseName), _) -->
  410    !,
  411    [ '~w <no source>'-[ClauseName] ].
  412where_no_goal(clause(Clause, PC), Options) -->
  413    { nonvar(Clause),
  414      !,
  415      clause_where(false, Clause, PC, Where, Options)
  416    },
  417    where_no_goal(Where, Options).
  418where_no_goal(meta_call, _) -->
  419    [ '<meta call>' ].
  420
  421where_goal(foreign(_), _) -->
  422    [ ' <foreign>'-[] ],
  423    !.
  424where_goal(pred_line(_PredName, File:Line), _) -->
  425    !,
  426    [ ' at ', url(File:Line) ].
  427where_goal(clause_name(ClauseName), _) -->
  428    !,
  429    [ '~w <no source>'-[ClauseName] ].
  430where_goal(clause(Clause, PC), Options) -->
  431    { nonvar(Clause),
  432      !,
  433      clause_where(false, Clause, PC, Where, Options)
  434    },
  435    where_goal(Where, Options).
  436where_goal(clause(Clause, _PC), _) -->
  437    { clause_property(Clause, file(File)),
  438      clause_property(Clause, line_count(Line))
  439    },
  440    !,
  441    [ ' at ', url(File:Line) ].
  442where_goal(clause(Clause, _PC), _) -->
  443    { clause_name(Clause, ClauseName)
  444    },
  445    !,
  446    [ ' ~w <no source>'-[ClauseName] ].
  447where_goal(_, _) -->
  448    [].
  449
  450level(Level) -->
  451    [ '~|~t[~D]~6+ '-[Level] ].
  452
  453warn_nodebug(Backtrace) -->
  454    { contiguous(Backtrace) },
  455    !.
  456warn_nodebug(_Backtrace) -->
  457    [ nl,nl,
  458      'Note: some frames are missing due to last-call optimization.'-[], nl,
  459      'Re-run your program in debug mode (:- debug.) to get more detail.'-[]
  460    ].
  461
  462contiguous([frame(D0,_,_)|Frames]) :-
  463    contiguous(Frames, D0).
  464
  465contiguous([], _).
  466contiguous([frame(D1,_,_)|Frames], D0) :-
  467    D1 =:= D0-1,
  468    contiguous(Frames, D1).
 clause_predicate_name(+ClauseRef, -Predname) is det
Produce a name (typically Functor/Arity) for a predicate to which Clause belongs.
  476clause_predicate_name(Clause, PredName) :-
  477    user:prolog_clause_name(Clause, PredName),
  478    !.
  479clause_predicate_name(Clause, PredName) :-
  480    nth_clause(Head, _N, Clause),
  481    !,
  482    predicate_name(user:Head, PredName).
 backtrace(+MaxDepth)
Get and print a stacktrace to the user_error stream.
  489backtrace(MaxDepth) :-
  490    get_prolog_backtrace_lc(MaxDepth, Stack, []),
  491    print_prolog_backtrace(user_error, Stack).
  492
  493
  494subgoal_position(ClauseRef, PC, File, CharA, CharZ) :-
  495    debug(backtrace, 'Term-position in ~p at PC=~w:', [ClauseRef, PC]),
  496    clause_info(ClauseRef, File, TPos, _),
  497    '$clause_term_position'(ClauseRef, PC, List),
  498    debug(backtrace, '\t~p~n', [List]),
  499    find_subgoal(List, TPos, PosTerm),
  500    arg(1, PosTerm, CharA),
  501    arg(2, PosTerm, CharZ).
 find_subgoal(+PosList, +TermPos, -SubGoalPos)
See also
- Same as find_subgoal/3 in trace.pl from the GUI tracer.
  507find_subgoal(_, Pos, Pos) :-
  508    var(Pos),
  509    !.
  510find_subgoal([A|T], term_position(_, _, _, _, PosL), SPos) :-
  511    nth1(A, PosL, Pos),
  512    !,
  513    find_subgoal(T, Pos, SPos).
  514find_subgoal([1|T], brace_term_position(_,_,Pos), SPos) :-
  515    !,
  516    find_subgoal(T, Pos, SPos).
  517find_subgoal(List, parentheses_term_position(_,_,Pos), SPos) :-
  518    !,
  519    find_subgoal(List, Pos, SPos).
  520find_subgoal(_, Pos, Pos).
 lineno(+File, +Char, -Line)
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.
  529lineno(File, Char, Line) :-
  530    setup_call_cleanup(
  531        ( prolog_clause:try_open_source(File, Fd),
  532          set_stream(Fd, newline(detect))
  533        ),
  534        lineno_(Fd, Char, Line),
  535        close(Fd)).
  536
  537lineno_(Fd, Char, L) :-
  538    stream_property(Fd, position(Pos)),
  539    stream_position_data(char_count, Pos, C),
  540    C > Char,
  541    !,
  542    stream_position_data(line_count, Pos, L0),
  543    L is L0-1.
  544lineno_(Fd, Char, L) :-
  545    skip(Fd, 0'\n),
  546    lineno_(Fd, Char, L).
  547
  548
  549		 /*******************************
  550		 *          CHOICEPOINTS	*
  551		 *******************************/
 print_last_choicepoint is det
Print details on the last open choice point.
  557print_last_choicepoint :-
  558    prolog_current_choice(ChI0),         % Choice in print_last_choicepoint/0
  559    prolog_choice_attribute(ChI0, parent, ChI1),
  560    print_last_choicepoint(ChI1, []).
  561print_last_choicepoint.
 print_last_choicepoint(+ChoiceRef, +Options) is det
  565print_last_choicepoint(ChI1, Options) :-
  566    real_choice(ChI1, ChI),
  567    prolog_choice_attribute(ChI, frame, F),
  568    prolog_frame_attribute(F, goal, Goal),
  569    Goal \= '$execute_goal2'(_,_,_),     % Toplevel REPL choicepoint
  570    !,
  571    option(message_level(Level), Options, warning),
  572    get_prolog_backtrace(2, [_|Stack], [frame(F)]),
  573    (   predicate_property(Goal, foreign)
  574    ->  print_message(Level, choicepoint(foreign(Goal), Stack))
  575    ;   prolog_frame_attribute(F, clause, Clause),
  576        (   prolog_choice_attribute(ChI, pc, PC)
  577        ->  Ctx = jump(PC)
  578        ;   prolog_choice_attribute(ChI, clause, Next)
  579        ->  Ctx = clause(Next)
  580        ),
  581        print_message(Level, choicepoint(clause(Goal, Clause, Ctx), Stack))
  582    ).
  583print_last_choicepoint(_, _).
  584
  585real_choice(Ch0, Ch) :-
  586    prolog_choice_attribute(Ch0, type, Type),
  587    dummy_type(Type),
  588    !,
  589    prolog_choice_attribute(Ch0, parent, Ch1),
  590    real_choice(Ch1, Ch).
  591real_choice(Ch, Ch).
  592
  593dummy_type(debug).
  594dummy_type(none).
  595
  596prolog:message(choicepoint(Choice, Stack)) -->
  597    choice(Choice),
  598    [ nl, 'Called from', nl ],
  599    message(Stack).
  600
  601choice(foreign(Goal)) -->
  602    success_goal(Goal, 'a foreign choice point').
  603choice(clause(Goal, ClauseRef, clause(Next))) -->
  604    success_goal(Goal, 'a choice point in alternate clause'),
  605    [ nl ],
  606    [ '  ' ], clause_descr(ClauseRef), [': clause succeeded', nl],
  607    [ '  ' ], clause_descr(Next),      [': next candidate clause' ].
  608choice(clause(Goal, ClauseRef, jump(PC))) -->
  609    { clause_where(false, ClauseRef, PC, Where,
  610                   [subgoal_positions(true)])
  611    },
  612    success_goal(Goal, 'an in-clause choice point'),
  613    [ nl, '  ' ],
  614    where_no_goal(Where).
  615
  616success_goal(Goal, Reason) -->
  617    [ ansi(code, '~p', [Goal]),
  618      ' left ~w (after success)'-[Reason]
  619    ].
  620
  621where_no_goal(pred_line(_PredName, File:Line)) -->
  622    !,
  623    [ url(File:Line) ].
  624where_no_goal(clause_name(ClauseName)) -->
  625    !,
  626    [ '~w <no source>'-[ClauseName] ].
  627
  628clause_descr(ClauseRef) -->
  629    { clause_property(ClauseRef, file(File)),
  630      clause_property(ClauseRef, line_count(Line))
  631    },
  632    !,
  633    [ url(File:Line) ].
  634clause_descr(ClauseRef) -->
  635    { clause_name(ClauseRef, Name)
  636    },
  637    [ '~w'-[Name] ].
  638
  639
  640                 /*******************************
  641                 *        DECORATE ERRORS       *
  642                 *******************************/
 prolog_stack:stack_guard(+PI) is semidet
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.
  678:- multifile
  679    user:prolog_exception_hook/4.  680:- dynamic
  681    user:prolog_exception_hook/4.  682
  683user:prolog_exception_hook(error(E, context(Ctx0,Msg)),
  684                           error(E, context(prolog_stack(Stack),Msg)),
  685                           Fr, GuardSpec) :-
  686    current_prolog_flag(backtrace, true),
  687    \+ is_stack(Ctx0, _Frames),
  688    (   atom(GuardSpec)
  689    ->  debug(backtrace, 'Got uncaught (guard = ~q) exception ~p (Ctx0=~p)',
  690              [GuardSpec, E, Ctx0]),
  691        stack_guard(GuardSpec),
  692        Guard = GuardSpec
  693    ;   prolog_frame_attribute(GuardSpec, predicate_indicator, Guard),
  694        debug(backtrace, 'Got exception ~p (Ctx0=~p, Catcher=~p)',
  695              [E, Ctx0, Guard]),
  696        stack_guard(Guard)
  697    ),
  698    (   current_prolog_flag(backtrace_depth, Depth)
  699    ->  Depth > 0
  700    ;   Depth = 20                  % Thread created before lib was loaded
  701    ),
  702    get_prolog_backtrace(Depth, Stack0,
  703                         [ frame(Fr),
  704                           guard(Guard)
  705                         ]),
  706    debug(backtrace, 'Stack = ~p', [Stack0]),
  707    clean_stack(Stack0, Stack1),
  708    join_stacks(Ctx0, Stack1, Stack).
  709
  710clean_stack(List, List) :-
  711    stack_guard(X), var(X),
  712    !.      % Do not stop if we catch all
  713clean_stack(List, Clean) :-
  714    clean_stack2(List, Clean).
  715
  716clean_stack2([], []).
  717clean_stack2([H|_], [H]) :-
  718    guard_frame(H),
  719    !.
  720clean_stack2([H|T0], [H|T]) :-
  721    clean_stack2(T0, T).
  722
  723guard_frame(frame(_,clause(ClauseRef, _, _))) :-
  724    nth_clause(M:Head, _, ClauseRef),
  725    functor(Head, Name, Arity),
  726    stack_guard(M:Name/Arity).
  727
  728join_stacks(Ctx0, Stack1, Stack) :-
  729    nonvar(Ctx0),
  730    Ctx0 = prolog_stack(Stack0),
  731    is_list(Stack0), !,
  732    append(Stack0, Stack1, Stack).
  733join_stacks(_, Stack, Stack).
 stack_guard(+Reason) is semidet
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.
  745stack_guard(none).
  746stack_guard(system:catch_with_backtrace/3).
  747
  748
  749                 /*******************************
  750                 *           MESSAGES           *
  751                 *******************************/
  752
  753:- multifile
  754    prolog:message//1.  755
  756prolog:message(error(Error, context(Stack, Message))) -->
  757    { Message \== 'DWIM could not correct goal',
  758      is_stack(Stack, Frames)
  759    },
  760    !,
  761    '$messages':translate_message(error(Error, context(_, Message))),
  762    [ nl, 'In:', nl ],
  763    (   {is_list(Frames)}
  764    ->  message(Frames)
  765    ;   ['~w'-[Frames]]
  766    ).
  767
  768is_stack(Stack, Frames) :-
  769    nonvar(Stack),
  770    Stack = prolog_stack(Frames)