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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        jan@swi-prolog.org
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2021-2022, SWI-Prolog Solutions b.v.
    7    All rights reserved.
    8
    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
   20
   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   25    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   26    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   27    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   28    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   29    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   31    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   32    POSSIBILITY OF SUCH DAMAGE.
   33*/
   34
   35:- module(prolog_debug_tools,
   36          [ (spy)/1,                % :Spec (some users tend to define these as
   37            (nospy)/1,              % :Spec  an operator)
   38            nospyall/0,
   39            debugging/0,
   40            trap/1,                 % +Exception
   41            notrap/1                % +Exception
   42          ]).   43:- use_module(library(broadcast), [broadcast/1]).   44:- autoload(library(edinburgh), [debug/0]).   45:- autoload(library(gensym), [gensym/2]).   46
   47:- set_prolog_flag(generate_debug_info, false).

User level debugging tools

This library provides tools to control the Prolog debuggers. Traditionally this code was built-in. Because these tools are only required in (interactive) debugging sessions they have been moved into the library. */

 prolog:debug_control_hook(+Action)
Allow user-hooks in the Prolog debugger interaction. See the calls below for the provided hooks. We use a single predicate with action argument to avoid an uncontrolled poliferation of hooks.
   63:- multifile
   64    prolog:debug_control_hook/1.    % +Action
   65
   66:- meta_predicate
   67    spy(:),
   68    nospy(:).
 spy :Spec is det
 nospy :Spec is det
 nospyall is det
Set/clear spy-points. A successfully set or cleared spy-point is reported using print_message/2, level informational, with one of the following terms, where Spec is of the form M:Head.
See also
- spy/1 and nospy/1 call the hook prolog:debug_control_hook/1 to allow for alternative specifications of the thing to debug.
   85spy(Spec) :-
   86    '$notrace'(spy_(Spec)).
   87
   88spy_(_:X) :-
   89    var(X),
   90    throw(error(instantiation_error, _)).
   91spy_(_:[]) :- !.
   92spy_(M:[H|T]) :-
   93    !,
   94    spy(M:H),
   95    spy(M:T).
   96spy_(Spec) :-
   97    prolog:debug_control_hook(spy(Spec)),
   98    !.
   99spy_(Spec) :-
  100    '$find_predicate'(Spec, Preds),
  101    '$member'(PI, Preds),
  102        pi_to_head(PI, Head),
  103        '$define_predicate'(Head),
  104        '$spy'(Head),
  105    fail.
  106spy_(_).
  107
  108nospy(Spec) :-
  109    '$notrace'(nospy_(Spec)).
  110
  111nospy_(_:X) :-
  112    var(X),
  113    throw(error(instantiation_error, _)).
  114nospy_(_:[]) :- !.
  115nospy_(M:[H|T]) :-
  116    !,
  117    nospy(M:H),
  118    nospy(M:T).
  119nospy_(Spec) :-
  120    prolog:debug_control_hook(nospy(Spec)),
  121    !.
  122nospy_(Spec) :-
  123    '$find_predicate'(Spec, Preds),
  124    '$member'(PI, Preds),
  125         pi_to_head(PI, Head),
  126        '$nospy'(Head),
  127    fail.
  128nospy_(_).
  129
  130nospyall :-
  131    '$notrace'(nospyall_).
  132
  133nospyall_ :-
  134    prolog:debug_control_hook(nospyall),
  135    fail.
  136nospyall_ :-
  137    spy_point(Head),
  138        '$nospy'(Head),
  139    fail.
  140nospyall_.
  141
  142pi_to_head(M:PI, M:Head) :-
  143    !,
  144    pi_to_head(PI, Head).
  145pi_to_head(Name/Arity, Head) :-
  146    functor(Head, Name, Arity).
 debugging is det
Report current status of the debugger.
  152debugging :-
  153    '$notrace'(debugging_).
  154
  155debugging_ :-
  156    prolog:debug_control_hook(debugging),
  157    !.
  158debugging_ :-
  159    (   current_prolog_flag(debug, true)
  160    ->  print_message(informational, debugging(on)),
  161        findall(H, spy_point(H), SpyPoints),
  162        print_message(informational, spying(SpyPoints))
  163    ;   print_message(informational, debugging(off))
  164    ),
  165    trapping.
  166
  167spy_point(Module:Head) :-
  168    current_predicate(_, Module:Head),
  169    '$get_predicate_attribute'(Module:Head, spy, 1),
  170    \+ predicate_property(Module:Head, imported_from(_)).
  171
  172
  173		 /*******************************
  174		 *           EXCEPTIONS		*
  175		 *******************************/
 trap(+Formal) is det
 notrap(+Formal) is det
Install a trap on error(Formal, Context) exceptions that unify. The tracer is started when a matching exception is raised. This predicate enables debug mode using debug/0 to get more context about the exception. Even with debug mode disabled exceptions are still trapped and thus one may call nodebug/0 to run in normal mode after installing a trap. Exceptions are trapped in any thread. Debug mode is only enabled in the calling thread. To enable debug mode in all threads use tdebug/0.

Calling debugging/0 lists the enabled traps. The predicate notrap/1 removes matching (unifying) traps.

In many cases debugging an exception that is caught is as simple as below (assuming run/0 starts your program).

?- gtrap(_).
?- run.
See also
- gtrap/1 to trap using the graphical debugger.
- Edit exceptions menu in PceEmacs and the graphical debugger that provide a graphical frontend to trap exceptions.
  204:- dynamic
  205    exception/4,                    % Name, Term, NotCaught, Caught
  206    installed/1.                    % ClauseRef
  207
  208trap(Error) :-
  209    '$notrace'(trap_(Error)).
  210
  211trap_(Error) :-
  212    gensym(ex, Rule),
  213    asserta(exception(Rule, error(Error, _), true, true)),
  214    print_message(informational, trap(Rule, error(Error, _), true, true)),
  215    install_exception_hook,
  216    debug.
  217
  218notrap(Error) :-
  219    '$notrace'(notrap_(Error)).
  220
  221notrap_(Error) :-
  222    Exception = error(Error, _),
  223    findall(exception(Name, Exception, NotCaught, Caught),
  224            retract(exception(Name, error(Error, _), Caught, NotCaught)),
  225            Trapping),
  226    print_message(informational, notrap(Trapping)).
  227
  228
  229trapping :-
  230    findall(exception(Name, Term, NotCaught, Caught),
  231            exception(Name, Term, NotCaught, Caught),
  232            Trapping),
  233    print_message(information, trapping(Trapping)).
  234
  235:- dynamic
  236    user:prolog_exception_hook/4.
 exception_hook(+ExIn, -ExOut, +Frame, +Catcher) is failure
Trap exceptions and consider whether or not to start the tracer.
  242:- public exception_hook/4.  243
  244exception_hook(Ex, Ex, _Frame, Catcher) :-
  245    thread_self(Me),
  246    thread_property(Me, debug(true)),
  247    broadcast(debug(exception(Ex))),
  248    exception(_, Ex, NotCaught, Caught),
  249    !,
  250    (   Caught == true
  251    ->  true
  252    ;   Catcher == none,
  253        NotCaught == true
  254    ),
  255    trace, fail.
 install_exception_hook
Make sure our handler is the first of the hook predicate.
  262install_exception_hook :-
  263    installed(Ref),
  264    (   nth_clause(_, I, Ref)
  265    ->  I == 1, !                   % Ok, we are the first
  266    ;   retractall(installed(Ref)),
  267        erase(Ref),                 % Someone before us!
  268        fail
  269    ).
  270install_exception_hook :-
  271    asserta((user:prolog_exception_hook(Ex, Out, Frame, Catcher) :-
  272                    exception_hook(Ex, Out, Frame, Catcher)), Ref),
  273    assert(installed(Ref)).
  274
  275
  276		 /*******************************
  277		 *            MESSAGES		*
  278		 *******************************/
  279
  280:- multifile
  281    prolog:message//1.  282
  283prolog:message(trapping([])) -->
  284    [ 'No exception traps'-[] ].
  285prolog:message(trapping(Trapping)) -->
  286    [ 'Exception traps on'-[], nl ],
  287    trapping(Trapping).
  288prolog:message(trap(_Rule, Error, _Caught, _NotCaught)) -->
  289    [ 'Installed trap for exception '-[] ],
  290    exception(Error),
  291    [ nl ].
  292prolog:message(notrap([])) -->
  293    [ 'No matching traps'-[] ].
  294prolog:message(notrap(Trapping)) -->
  295    [ 'Removed traps from exceptions'-[], nl ],
  296    trapping(Trapping).
  297
  298trapping([]) --> [].
  299trapping([exception(_Rule, Error, _Caught, _NotCaught)|T]) -->
  300    [ '  '-[] ],
  301    exception(Error),
  302    [ nl ],
  303    trapping(T).
  304
  305exception(Term) -->
  306    { copy_term(Term, T2),
  307      numbervars(T2, 0, _, [singletons(true)])
  308    },
  309    [ '~p'-[T2] ]