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)  2004-2016, University of Amsterdam
    7                              VU University Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   29    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   30    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(prolog_stack,
   37          [ get_prolog_backtrace/2,     % +MaxDepth, -Stack
   38            get_prolog_backtrace/3,     % +Frame, +MaxDepth, -Stack
   39            prolog_stack_frame_property/2, % +Frame, ?Property
   40            print_prolog_backtrace/2,   % +Stream, +Stack
   41            print_prolog_backtrace/3,   % +Stream, +Stack, +Options
   42            backtrace/1                 % +MaxDepth
   43          ]).   44:- use_module(library(prolog_clause)).   45:- use_module(library(debug)).   46:- use_module(library(error)).   47:- use_module(library(lists)).   48:- use_module(library(option)).   49
   50:- dynamic stack_guard/1.   51:- multifile stack_guard/1.   52
   53:- predicate_options(print_prolog_backtrace/3, 3,
   54                     [ subgoal_positions(boolean)
   55                     ]).   56
   57/** <module> Examine the Prolog stack
   58
   59This module defines  high-level  primitives   for  examining  the Prolog
   60stack,  primarily  intended  to  support   debugging.  It  provides  the
   61following functionality:
   62
   63    * get_prolog_backtrace/2 gets a Prolog representation of the
   64    Prolog stack.  This can be used for printing, but also to enrich
   65    exceptions using prolog_exception_hook/4.  Decorating exceptions
   66    is provided by this library and controlled by the hook
   67    stack_guard/1.
   68
   69    * print_prolog_backtrace/2 prints a backtrace as returned by
   70    get_prolog_backtrace/2
   71
   72    * The shorthand backtrace/1 fetches and prints a backtrace.
   73
   74This library may be enabled by default to improve interactive debugging,
   75for example by adding the lines below   to  your ~/swiplrc (swipl.ini in
   76Windows) to decorate uncaught exceptions:
   77
   78  ==
   79  :- use_module(library(prolog_stack)).
   80  ==
   81
   82@bug    Use of this library may negatively impact performance of
   83        applications that process (error-)exceptions frequently
   84        as part of their normal processing.
   85*/
   86
   87:- create_prolog_flag(backtrace,            true, [type(boolean), keep(true)]).   88:- create_prolog_flag(backtrace_depth,      20,   [type(integer), keep(true)]).   89:- create_prolog_flag(backtrace_goal_depth, 3,    [type(integer), keep(true)]).   90:- create_prolog_flag(backtrace_show_lines, true, [type(boolean), keep(true)]).   91
   92%!  get_prolog_backtrace(+MaxDepth, -Backtrace) is det.
   93%!  get_prolog_backtrace(+MaxDepth, -Backtrace, +Options) is det.
   94%
   95%   Obtain a backtrace from the current location. The backtrace is a
   96%   list of frames. Each  frame  is  an   opaque  term  that  can be
   97%   inspected using the predicate  prolog_stack_frame_property/2 can
   98%   be used to extract  information  from   these  frames.  Most use
   99%   scenarios will pass the stack   to print_prolog_backtrace/2. The
  100%   following options are provided:
  101%
  102%     * frame(Frame)
  103%     Start at Frame instead of the current frame.
  104%     * goal_depth(+Depth)
  105%     If Depth > 0, include a shallow copy of the goal arguments
  106%     into the stack.  Default is set by the Prolog flag
  107%     =backtrace_goal_depth=, set to =2= initially, showing the
  108%     goal and toplevel of any argument.
  109%
  110%   @param Frame is the frame to start from. See prolog_current_frame/1.
  111%   @param MaxDepth defines the maximum number of frames returned.
  112%   @compat get_prolog_backtrace/3 used to have the parameters
  113%   +Frame, +MaxDepth, -Backtrace. A call that matches this
  114%   signature is mapped to get_prolog_backtrace(MaxDepth, Backtrace,
  115%   [frame(Frame)]).
  116
  117get_prolog_backtrace(MaxDepth, Stack) :-
  118    get_prolog_backtrace(MaxDepth, Stack, []).
  119
  120get_prolog_backtrace(Fr, MaxDepth, Stack) :-
  121    integer(Fr), integer(MaxDepth), var(Stack),
  122    !,
  123    get_prolog_backtrace_lc(MaxDepth, Stack, [frame(Fr)]),
  124    nlc.
  125get_prolog_backtrace(MaxDepth, Stack, Options) :-
  126    get_prolog_backtrace_lc(MaxDepth, Stack, Options),
  127    nlc.            % avoid last-call-optimization, such that
  128                        % the top of the stack is always a nice Prolog
  129                        % frame
  130
  131nlc.
  132
  133get_prolog_backtrace_lc(MaxDepth, Stack, Options) :-
  134    (   option(frame(Fr), Options)
  135    ->  PC = call
  136    ;   prolog_current_frame(Fr0),
  137        prolog_frame_attribute(Fr0, pc, PC),
  138        prolog_frame_attribute(Fr0, parent, Fr)
  139    ),
  140    (   option(goal_term_depth(GoalDepth), Options)
  141    ->  true
  142    ;   current_prolog_flag(backtrace_goal_depth, GoalDepth)
  143    ),
  144    must_be(nonneg, GoalDepth),
  145    backtrace(MaxDepth, Fr, PC, GoalDepth, Stack).
  146
  147backtrace(0, _, _, _, []) :- !.
  148backtrace(MaxDepth, Fr, PC, GoalDepth,
  149          [frame(Level, Where, Goal)|Stack]) :-
  150    prolog_frame_attribute(Fr, level, Level),
  151    (   PC == foreign
  152    ->  prolog_frame_attribute(Fr, predicate_indicator, Pred),
  153        Where = foreign(Pred)
  154    ;   PC == call
  155    ->  prolog_frame_attribute(Fr, predicate_indicator, Pred),
  156        Where = call(Pred)
  157    ;   prolog_frame_attribute(Fr, clause, Clause)
  158    ->  Where = clause(Clause, PC)
  159    ;   Where = meta_call
  160    ),
  161    (   Where == meta_call
  162    ->  Goal = 0
  163    ;   copy_goal(GoalDepth, Fr, Goal)
  164    ),
  165    (   prolog_frame_attribute(Fr, pc, PC2)
  166    ->  true
  167    ;   PC2 = foreign
  168    ),
  169    (   prolog_frame_attribute(Fr, parent, Parent),
  170        more_stack(Parent)
  171    ->  D2 is MaxDepth - 1,
  172        backtrace(D2, Parent, PC2, GoalDepth, Stack)
  173    ;   Stack = []
  174    ).
  175
  176more_stack(Parent) :-
  177    prolog_frame_attribute(Parent, predicate_indicator, PI),
  178    \+ ( PI = '$toplevel':G,
  179         G \== (toplevel_call/1)
  180       ),
  181    !.
  182more_stack(_) :-
  183    current_prolog_flag(break_level, Break),
  184    Break >= 1.
  185
  186%!  copy_goal(+TermDepth, +Frame, -Goal) is det.
  187%
  188%   Create a shallow copy of the frame's  goal to help debugging. In
  189%   addition to shallow copying, high-arity   terms  are represented
  190%   as below.  Currently the 16 first arguments are hardcoded.
  191%
  192%     ==
  193%     name(A1, ..., A16, <skipped Skipped of Arity>, An)
  194%     ==
  195
  196copy_goal(0, _, 0) :- !.                        % 0 is not a valid goal
  197copy_goal(D, Fr, Goal) :-
  198    prolog_frame_attribute(Fr, goal, Goal0),
  199    (   Goal0 = Module:Goal1
  200    ->  copy_term_limit(D, Goal1, Goal2),
  201        (   hidden_module(Module)
  202        ->  Goal = Goal2
  203        ;   Goal = Module:Goal2
  204        )
  205    ;   copy_term_limit(D, Goal0, Goal)
  206    ).
  207
  208hidden_module(system).
  209hidden_module(user).
  210
  211copy_term_limit(0, In, '...') :-
  212    compound(In),
  213    !.
  214copy_term_limit(N, In, Out) :-
  215    is_dict(In),
  216    !,
  217    dict_pairs(In, Tag, PairsIn),
  218    N2 is N - 1,
  219    MaxArity = 16,
  220    copy_pairs(PairsIn, N2, MaxArity, PairsOut),
  221    dict_pairs(Out, Tag, PairsOut).
  222copy_term_limit(N, In, Out) :-
  223    compound(In),
  224    !,
  225    compound_name_arity(In, Functor, Arity),
  226    N2 is N - 1,
  227    MaxArity = 16,
  228    (   Arity =< MaxArity
  229    ->  compound_name_arity(Out, Functor, Arity),
  230        copy_term_args(0, Arity, N2, In, Out)
  231    ;   OutArity is MaxArity+2,
  232        compound_name_arity(Out, Functor, OutArity),
  233        copy_term_args(0, MaxArity, N2, In, Out),
  234        SkipArg is MaxArity+1,
  235        Skipped is Arity - MaxArity - 1,
  236        format(atom(Msg), '<skipped ~D of ~D>', [Skipped, Arity]),
  237        arg(SkipArg, Out, Msg),
  238        arg(Arity, In, InA),
  239        arg(OutArity, Out, OutA),
  240        copy_term_limit(N2, InA, OutA)
  241    ).
  242copy_term_limit(_, In, Out) :-
  243    copy_term_nat(In, Out).
  244
  245copy_term_args(I, Arity, Depth, In, Out) :-
  246    I < Arity,
  247    !,
  248    I2 is I + 1,
  249    arg(I2, In, InA),
  250    arg(I2, Out, OutA),
  251    copy_term_limit(Depth, InA, OutA),
  252    copy_term_args(I2, Arity, Depth, In, Out).
  253copy_term_args(_, _, _, _, _).
  254
  255copy_pairs([], _, _, []) :- !.
  256copy_pairs(Pairs, _, 0, ['<skipped>'-Skipped]) :-
  257    !,
  258    length(Pairs, Skipped).
  259copy_pairs([K-V0|T0], N, MaxArity, [K-V|T]) :-
  260    copy_term_limit(N, V0, V),
  261    MaxArity1 is MaxArity - 1,
  262    copy_pairs(T0, N, MaxArity1, T).
  263
  264
  265%!  prolog_stack_frame_property(+Frame, ?Property) is nondet.
  266%
  267%   True when Property is a property of   Frame. Frame is an element
  268%   of a stack-trace as produced by get_prolog_backtrace/2.  Defined
  269%   properties are:
  270%
  271%     * level(Level)
  272%     * predicate(PI)
  273%     * location(File:Line)
  274
  275prolog_stack_frame_property(frame(Level,_,_), level(Level)).
  276prolog_stack_frame_property(frame(_,Where,_), predicate(PI)) :-
  277    frame_predicate(Where, PI).
  278prolog_stack_frame_property(frame(_,clause(Clause,PC),_), location(File:Line)) :-
  279    subgoal_position(Clause, PC, File, CharA, _CharZ),
  280    File \= @(_),                   % XPCE Object reference
  281    lineno(File, CharA, Line).
  282prolog_stack_frame_property(frame(_,_,_,Goal), goal(Goal)) :-
  283    Goal \== 0.
  284
  285
  286frame_predicate(foreign(PI), PI).
  287frame_predicate(call(PI), PI).
  288frame_predicate(clause(Clause, _PC), PI) :-
  289    clause_property(Clause, PI).
  290
  291default_backtrace_options(Options) :-
  292    (   current_prolog_flag(backtrace_show_lines, true)
  293    ->  Options = []
  294    ;   Options = [subgoal_positions(false)]
  295    ).
  296
  297%!  print_prolog_backtrace(+Stream, +Backtrace) is det.
  298%!  print_prolog_backtrace(+Stream, +Backtrace, +Options) is det.
  299%
  300%   Print a stacktrace in human readable form to Stream.
  301%   Options is an option list that accepts:
  302%
  303%       * subgoal_positions(+Boolean)
  304%       If =true=, print subgoal line numbers.  The default depends
  305%       on the Prolog flag =backtrace_show_lines=.
  306%
  307%   @arg Backtrace is a list of frame(Depth,Location,Goal) terms.
  308
  309print_prolog_backtrace(Stream, Backtrace) :-
  310    print_prolog_backtrace(Stream, Backtrace, []).
  311
  312print_prolog_backtrace(Stream, Backtrace, Options) :-
  313    default_backtrace_options(DefOptions),
  314    merge_options(Options, DefOptions, FinalOptions),
  315    phrase(message(Backtrace, FinalOptions), Lines),
  316    print_message_lines(Stream, '', Lines).
  317
  318:- public                               % Called from some handlers
  319    message//1.  320
  321message(Backtrace) -->
  322    {default_backtrace_options(Options)},
  323    message(Backtrace, Options).
  324
  325message(Backtrace, Options) -->
  326    message_frames(Backtrace, Options),
  327    warn_nodebug(Backtrace).
  328
  329message_frames([], _) -->
  330    [].
  331message_frames([H|T], Options) -->
  332    message_frames(H, Options),
  333    (   {T == []}
  334    ->  []
  335    ;   [nl],
  336        message_frames(T, Options)
  337    ).
  338
  339message_frames(frame(Level, Where, 0), Options) -->
  340    !,
  341    level(Level),
  342    where_no_goal(Where, Options).
  343message_frames(frame(Level, _Where, '$toplevel':toplevel_call(_)), _) -->
  344    !,
  345    level(Level),
  346    [ '<user>'-[] ].
  347message_frames(frame(Level, Where, Goal), Options) -->
  348    level(Level),
  349    [ '~q'-[Goal] ],
  350    where_goal(Where, Options).
  351
  352where_no_goal(foreign(PI), _) -->
  353    [ '~w <foreign>'-[PI] ].
  354where_no_goal(call(PI), _) -->
  355    [ '~w'-[PI] ].
  356where_no_goal(clause(Clause, PC), Options) -->
  357    { option(subgoal_positions(true), Options, true),
  358      subgoal_position(Clause, PC, File, CharA, _CharZ),
  359      File \= @(_),                 % XPCE Object reference
  360      lineno(File, CharA, Line),
  361      clause_predicate_name(Clause, PredName)
  362    },
  363    !,
  364    [ '~w at ~w:~d'-[PredName, File, Line] ].
  365where_no_goal(clause(Clause, _PC), _) -->
  366    { clause_property(Clause, file(File)),
  367      clause_property(Clause, line_count(Line)),
  368      clause_predicate_name(Clause, PredName)
  369    },
  370    !,
  371    [ '~w at ~w:~d'-[PredName, File, Line] ].
  372where_no_goal(clause(Clause, _PC), _) -->
  373    { clause_name(Clause, ClauseName)
  374    },
  375    [ '~w <no source>'-[ClauseName] ].
  376where_no_goal(meta_call, _) -->
  377    [ '<meta call>' ].
  378
  379where_goal(foreign(_), _) -->
  380    [ ' <foreign>'-[] ],
  381    !.
  382where_goal(clause(Clause, PC), Options) -->
  383    { option(subgoal_positions(true), Options, true),
  384      subgoal_position(Clause, PC, File, CharA, _CharZ),
  385      File \= @(_),                 % XPCE Object reference
  386      lineno(File, CharA, Line)
  387    },
  388    !,
  389    [ ' at ~w:~d'-[File, Line] ].
  390where_goal(clause(Clause, _PC), _) -->
  391    { clause_property(Clause, file(File)),
  392      clause_property(Clause, line_count(Line))
  393    },
  394    !,
  395    [ ' at ~w:~d'-[ File, Line] ].
  396where_goal(clause(Clause, _PC), _) -->
  397    { clause_name(Clause, ClauseName)
  398    },
  399    !,
  400    [ ' ~w <no source>'-[ClauseName] ].
  401where_goal(_, _) -->
  402    [].
  403
  404level(Level) -->
  405    [ '~|~t[~D]~6+ '-[Level] ].
  406
  407warn_nodebug(Backtrace) -->
  408    { contiguous(Backtrace) },
  409    !.
  410warn_nodebug(_Backtrace) -->
  411    [ nl,nl,
  412      'Note: some frames are missing due to last-call optimization.'-[], nl,
  413      'Re-run your program in debug mode (:- debug.) to get more detail.'-[]
  414    ].
  415
  416contiguous([frame(D0,_,_)|Frames]) :-
  417    contiguous(Frames, D0).
  418
  419contiguous([], _).
  420contiguous([frame(D1,_,_)|Frames], D0) :-
  421    D1 =:= D0-1,
  422    contiguous(Frames, D1).
  423
  424
  425%!  clause_predicate_name(+ClauseRef, -Predname) is det.
  426%
  427%   Produce a name (typically  Functor/Arity)   for  a  predicate to
  428%   which Clause belongs.
  429
  430clause_predicate_name(Clause, PredName) :-
  431    user:prolog_clause_name(Clause, PredName),
  432    !.
  433clause_predicate_name(Clause, PredName) :-
  434    nth_clause(Head, _N, Clause),
  435    !,
  436    predicate_name(user:Head, PredName).
  437
  438
  439%!  backtrace(+MaxDepth)
  440%
  441%   Get and print a stacktrace to the user_error stream.
  442
  443backtrace(MaxDepth) :-
  444    get_prolog_backtrace_lc(MaxDepth, Stack, []),
  445    print_prolog_backtrace(user_error, Stack).
  446
  447
  448subgoal_position(ClauseRef, PC, File, CharA, CharZ) :-
  449    debug(backtrace, 'Term-position in ~p at PC=~w:', [ClauseRef, PC]),
  450    clause_info(ClauseRef, File, TPos, _),
  451    '$clause_term_position'(ClauseRef, PC, List),
  452    debug(backtrace, '\t~p~n', [List]),
  453    find_subgoal(List, TPos, PosTerm),
  454    arg(1, PosTerm, CharA),
  455    arg(2, PosTerm, CharZ).
  456
  457find_subgoal([A|T], term_position(_, _, _, _, PosL), SPos) :-
  458    is_list(PosL),
  459    nth1(A, PosL, Pos),
  460    nonvar(Pos),
  461    !,
  462    find_subgoal(T, Pos, SPos).
  463find_subgoal([], Pos, Pos).
  464
  465
  466%!  lineno(+File, +Char, -Line)
  467%
  468%   Translate a character location to a line-number.
  469
  470lineno(File, Char, Line) :-
  471    setup_call_cleanup(
  472        ( open(File, read, Fd),
  473          set_stream(Fd, newline(detect))
  474        ),
  475        lineno_(Fd, Char, Line),
  476        close(Fd)).
  477
  478lineno_(Fd, Char, L) :-
  479    stream_property(Fd, position(Pos)),
  480    stream_position_data(char_count, Pos, C),
  481    C > Char,
  482    !,
  483    stream_position_data(line_count, Pos, L0),
  484    L is L0-1.
  485lineno_(Fd, Char, L) :-
  486    skip(Fd, 0'\n),
  487    lineno_(Fd, Char, L).
  488
  489
  490                 /*******************************
  491                 *        DECORATE ERRORS       *
  492                 *******************************/
  493
  494%!  prolog_stack:stack_guard(+PI) is semidet.
  495%
  496%   Dynamic multifile hook that is normally not defined. The hook is
  497%   called with PI equal to =none= if   the  exception is not caught
  498%   and with a fully qualified   (e.g., Module:Name/Arity) predicate
  499%   indicator of the predicate that called  catch/3 if the exception
  500%   is caught.
  501%
  502%   The exception is of the form error(Formal, ImplDef) and this
  503%   hook succeeds, ImplDef is unified to a term
  504%   context(prolog_stack(StackData), Message).  This context
  505%   information is used by the message printing system to print a
  506%   human readable representation of the stack when the exception
  507%   was raised.
  508%
  509%   For example, using a clause   stack_guard(none)  prints contexts
  510%   for uncaught exceptions only.  Using   a  clause  stack_guard(_)
  511%   prints a full  stack-trace  for  any   error  exception  if  the
  512%   exception   is   given    to     print_message/2.    See    also
  513%   library(http/http_error), which limits printing of exceptions to
  514%   exceptions in user-code called from the HTTP server library.
  515%
  516%   Details of the exception decoration is  controlled by two Prolog
  517%   flags:
  518%
  519%       * backtrace_depth
  520%       Integer that controls the maximum number of frames
  521%       collected.  Default is 20.  If a guard is specified, callers
  522%       of the guard are removed from the stack-trace.
  523%
  524%       * backtrace_show_lines
  525%       Boolean that indicates whether the library tries to find
  526%       line numbers for the calls.  Default is =true=.
  527
  528:- multifile
  529    user:prolog_exception_hook/4.  530:- dynamic
  531    user:prolog_exception_hook/4.  532
  533user:prolog_exception_hook(error(E, context(Ctx0,Msg)),
  534                           error(E, context(prolog_stack(Stack),Msg)),
  535                           Fr, Guard) :-
  536    current_prolog_flag(backtrace, true),
  537    (   atom(Guard)
  538    ->  debug(backtrace, 'Got uncaught (guard = ~q) exception ~p (Ctx0=~p)',
  539              [Guard, E, Ctx0]),
  540        stack_guard(Guard)
  541    ;   prolog_frame_attribute(Guard, predicate_indicator, PI),
  542        debug(backtrace, 'Got exception ~p (Ctx0=~p, Catcher=~p)',
  543              [E, Ctx0, PI]),
  544        stack_guard(PI)
  545    ),
  546    (   current_prolog_flag(backtrace_depth, Depth)
  547    ->  Depth > 0
  548    ;   Depth = 20                  % Thread created before lib was loaded
  549    ),
  550    get_prolog_backtrace(Fr, Depth, Stack0),
  551    debug(backtrace, 'Stack = ~p', [Stack0]),
  552    clean_stack(Stack0, Stack1),
  553    join_stacks(Ctx0, Stack1, Stack).
  554
  555clean_stack(List, List) :-
  556    stack_guard(X), var(X),
  557    !.      % Do not stop if we catch all
  558clean_stack(List, Clean) :-
  559    clean_stack2(List, Clean).
  560
  561clean_stack2([], []).
  562clean_stack2([H|_], [H]) :-
  563    guard_frame(H),
  564    !.
  565clean_stack2([H|T0], [H|T]) :-
  566    clean_stack2(T0, T).
  567
  568guard_frame(frame(_,clause(ClauseRef, _, _))) :-
  569    nth_clause(M:Head, _, ClauseRef),
  570    functor(Head, Name, Arity),
  571    stack_guard(M:Name/Arity).
  572
  573join_stacks(Ctx0, Stack1, Stack) :-
  574    nonvar(Ctx0),
  575    Ctx0 = prolog_stack(Stack0),
  576    is_list(Stack0), !,
  577    append(Stack0, Stack1, Stack).
  578join_stacks(_, Stack, Stack).
  579
  580
  581%!  stack_guard(+Reason) is semidet.
  582%
  583%   Dynamic multifile predicate. It is called  with `none`, `'C'` or
  584%   the predicate indicator of the   _guard_,  the predicate calling
  585%   catch/3. The exception must be of   _compatible_  with the shape
  586%   error(Formal, context(Stack, Msg)). The  default   is  to  catch
  587%   `none`, uncaught exceptions. `'C'`  implies   that  the callback
  588%   from C will handle the exception.
  589
  590stack_guard(none).
  591
  592
  593                 /*******************************
  594                 *           MESSAGES           *
  595                 *******************************/
  596
  597:- multifile
  598    prolog:message//1.  599
  600prolog:message(error(Error, context(Stack, Message))) -->
  601    { Message \== 'DWIM could not correct goal',
  602      is_stack(Stack, Frames)
  603    },
  604    !,
  605    '$messages':translate_message(error(Error, context(_, Message))),
  606    [ nl, 'In:', nl ],
  607    (   {is_list(Frames)}
  608    ->  message(Frames)
  609    ;   ['~w'-[Frames]]
  610    ).
  611
  612is_stack(Stack, Frames) :-
  613    nonvar(Stack),
  614    Stack = prolog_stack(Frames)