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)  2021-2023, 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).

 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.

• spy(Spec)
• nospy(Spec)

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    '$notrace'(debugging_).
  154
  155debugging_ :-
  156    prolog:debug_control_hook(debugging),
  157    !.
  158debugging_ :-
  159    (   current_prolog_flag(debug, true)
  160    ->  print_message(informational, debugging(on)),
  161        findall(H, spy_point(H), SpyPoints),
  162        print_message(informational, spying(SpyPoints))
  163    ;   print_message(informational, debugging(off))
  164    ),
  165    trapping,
  166    forall(debugging_hook, true).
  167
  168spy_point(Module:Head) :-
  169    current_predicate(_, Module:Head),
  170    '$get_predicate_attribute'(Module:Head, spy, 1),
  171    \+ predicate_property(Module:Head, imported_from(_)).

 debugging_hook

Multifile hook that is called as forall(debugging_hook, true) and that may be used to extend the information printed from other debugging libraries.

  179:- multifile debugging_hook/0.  180
  181
  182		 /*******************************
  183		 *           EXCEPTIONS		*
  184		 *******************************/

 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).

?- trap(_).
?- 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.

  213:- dynamic
  214    exception/4,                    % Name, Term, NotCaught, Caught
  215    installed/1.                    % ClauseRef
  216
  217trap(Error) :-
  218    '$notrace'(trap_(Error)).
  219
  220trap_(Error) :-
  221    gensym(ex, Rule),
  222    asserta(exception(Rule, error(Error, _), true, true)),
  223    print_message(informational, trap(Rule, error(Error, _), true, true)),
  224    install_exception_hook,
  225    debug.
  226
  227notrap(Error) :-
  228    '$notrace'(notrap_(Error)).
  229
  230notrap_(Error) :-
  231    Exception = error(Error, _),
  232    findall(exception(Name, Exception, NotCaught, Caught),
  233            retract(exception(Name, error(Error, _), Caught, NotCaught)),
  234            Trapping),
  235    print_message(informational, notrap(Trapping)).
  236
  237
  238trapping :-
  239    findall(exception(Name, Term, NotCaught, Caught),
  240            exception(Name, Term, NotCaught, Caught),
  241            Trapping),
  242    print_message(information, trapping(Trapping)).
  243
  244:- dynamic   prolog:prolog_exception_hook/5.  245:- multifile prolog:prolog_exception_hook/5.

 exception_hook(+ExIn, -ExOut, +Frame, +Catcher) is failure

Trap exceptions and consider whether or not to start the tracer.

  251:- public exception_hook/5.  252
  253exception_hook(Ex, Ex, _Frame, Catcher, _Debug) :-
  254    thread_self(Me),
  255    thread_property(Me, debug(true)),
  256    broadcast(debug(exception(Ex))),
  257    exception(_, Ex, NotCaught, Caught),
  258    !,
  259    (   Caught == true
  260    ->  true
  261    ;   Catcher == none,
  262        NotCaught == true
  263    ),
  264    trace, fail.

 install_exception_hook

Make sure our handler is the first of the hook predicate.

  271install_exception_hook :-
  272    installed(Ref),
  273    (   nth_clause(_, I, Ref)
  274    ->  I == 1, !                   % Ok, we are the first
  275    ;   retractall(installed(Ref)),
  276        erase(Ref),                 % Someone before us!
  277        fail
  278    ).
  279install_exception_hook :-
  280    asserta((prolog:prolog_exception_hook(Ex, Out, Frame, Catcher, Debug) :-
  281                    exception_hook(Ex, Out, Frame, Catcher, Debug)), Ref),
  282    assert(installed(Ref)).
  283
  284
  285		 /*******************************
  286		 *            MESSAGES		*
  287		 *******************************/
  288
  289:- multifile
  290    prolog:message//1.  291
  292prolog:message(trapping([])) -->
  293    [ 'No exception traps'-[] ].
  294prolog:message(trapping(Trapping)) -->
  295    [ 'Exception traps on'-[], nl ],
  296    trapping(Trapping).
  297prolog:message(trap(_Rule, Error, _Caught, _NotCaught)) -->
  298    [ 'Installed trap for exception '-[] ],
  299    exception(Error),
  300    [ nl ].
  301prolog:message(notrap([])) -->
  302    [ 'No matching traps'-[] ].
  303prolog:message(notrap(Trapping)) -->
  304    [ 'Removed traps from exceptions'-[], nl ],
  305    trapping(Trapping).
  306
  307trapping([]) --> [].
  308trapping([exception(_Rule, Error, _Caught, _NotCaught)|T]) -->
  309    [ '  '-[] ],
  310    exception(Error),
  311    [ nl ],
  312    trapping(T).
  313
  314exception(Term) -->
  315    { copy_term(Term, T2),
  316      numbervars(T2, 0, _, [singletons(true)])
  317    },
  318    [ '~p'-[T2] ]