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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)  1985-2021, University of Amsterdam,
    7                              VU University Amsterdam
    8                              SWI-Prolog Solutions b.v.
    9    All rights reserved.
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   13    are met:
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   24    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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   35*/
   36
   37:- module(prolog_explain,
   38          [ explain/1,
   39            explain/2
   40          ]).   41:- autoload(library(apply),[maplist/2,maplist/3]).   42:- autoload(library(lists),[flatten/2]).   43:- autoload(library(prolog_code), [pi_head/2]).   44
   45:- if(exists_source(library(pldoc/man_index))).   46:- autoload(library(pldoc/man_index), [man_object_property/2]).   47:- endif.

Describe Prolog Terms

The library(explain) describes prolog-terms. The most useful functionality is its cross-referencing function.

?- explain(subset(_,_)).
"subset(_, _)" is a compound term
    from 2-th clause of lists:subset/2
    Referenced from 46-th clause of prolog_xref:imported/3
    Referenced from 68-th clause of prolog_xref:imported/3
lists:subset/2 is a predicate defined in
    /staff/jan/lib/pl-5.6.17/library/lists.pl:307
    Referenced from 2-th clause of lists:subset/2
    Possibly referenced from 2-th clause of lists:subset/2

Note that PceEmacs can jump to definitions and gxref/0 can be used for an overview of dependencies. */

 explain(@Term) is det
Give an explanation on Term. The argument may be any Prolog data object. If the argument is an atom, a term of the form Name/Arity or a term of the form Module:Name/Arity, explain/1 describes the predicate as well as possible references to it. See also gxref/0.
   77explain(Item) :-
   78    explain(Item, Explanation),
   79    print_message(information, explain(Explanation)),
   80    fail.
   81explain(_).
   82
   83                /********************************
   84                *           BASIC TYPES         *
   85                *********************************/
 explain(@Term, -Explanation) is nondet
True when Explanation is an explanation of Term. The explaination is a list of elements that is printed using print_message(information, explain(Explanation)).
   93explain(Var, [isa(Var, 'unbound variable')]) :-
   94    var(Var),
   95    !.
   96explain(I, [isa(I, 'an integer')]) :-
   97    integer(I),
   98    !.
   99explain(F, [isa(F, 'a floating point number')]) :-
  100    float(F),
  101    !.
  102explain(Q, [isa(Q, 'a rational (Q) number')]) :-
  103    rational(Q),
  104    !.
  105explain(S, [isa(S, 'a string')]) :-
  106    string(S),
  107    !.
  108explain([], [isa([], 'a special constant denoting an empty list')]) :-
  109    !.
  110explain(A, [isa(A, 'an atom')]) :-
  111    atom(A).
  112explain(A, Explanation) :-
  113    atom(A),
  114    current_op(Pri, F, A),
  115    op_type(F, Type),
  116    Explanation = [ isa(A, 'a ~w (~w) operator of priority ~d'-[Type, F, Pri]) ].
  117explain(A, Explanation) :-
  118    atom(A),
  119    !,
  120    explain_atom(A, Explanation).
  121explain([H|T], Explanation) :-
  122    List = [H|T],
  123    is_list(T),
  124    !,
  125    length(List, L),
  126    (   Explanation = [ isa(List, 'a proper list with ~d elements'-[L]) ]
  127    ;   maplist(printable, List),
  128        Explanation = [ indent, 'Text is "~s"'-[List] ]
  129    ).
  130explain(List, Explanation) :-
  131    List = [_|_],
  132    !,
  133    length(List, L),
  134    !,
  135    Explanation = [isa(List, 'is a not-closed list with ~d elements'-[L])].
  136explain(Name//NTArity, Explanation) :-
  137    atom(Name),
  138    integer(NTArity),
  139    NTArity >= 0,
  140    !,
  141    Arity is NTArity + 2,
  142    explain(Name/Arity, Explanation).
  143explain(Name/Arity, Explanation) :-
  144    atom(Name),
  145    integer(Arity),
  146    Arity >= 0,
  147    !,
  148    functor(Head, Name, Arity),
  149    known_predicate(Module:Head),
  150    (   Module == system
  151    ->  true
  152    ;   \+ predicate_property(Module:Head, imported_from(_))
  153    ),
  154    explain_predicate(Module:Head, Explanation).
  155explain(Module:Name/Arity, Explanation) :-
  156    atom(Module), atom(Name), integer(Arity),
  157    !,
  158    functor(Head, Name, Arity),
  159    explain_predicate(Module:Head, Explanation).
  160explain(Module:Head, Explanation) :-
  161    callable(Head),
  162    !,
  163    explain_predicate(Module:Head, Explanation).
  164explain(Dict, Explanation) :-
  165    is_dict(Dict, Tag),
  166    !,
  167    Explanation = [isa(Dict, 'a dict with tag ~q'-[Tag]) ].
  168explain(Term, Explanation) :-
  169    compound(Term),
  170    compound_name_arity(Term, _Name, Arity),
  171    numbervars(Term, 0, _, [singletons(true)]),
  172    Explanation = [isa(Term, 'is a compound term with arity ~D'-[Arity])].
  173explain(Term, Explanation) :-
  174    explain_functor(Term, Explanation).
 known_predicate(:Head)
Succeeds if we know anything about this predicate. Undefined predicates are considered `known' for this purpose, so we can provide referenced messages on them.
  182known_predicate(M:Head) :-
  183    var(M),
  184    current_predicate(_, M2:Head),
  185    (   predicate_property(M2:Head, imported_from(M))
  186    ->  true
  187    ;   M = M2
  188    ),
  189    !.
  190known_predicate(Pred) :-
  191    predicate_property(Pred, undefined).
  192known_predicate(_:Head) :-
  193    functor(Head, Name, Arity),
  194    '$in_library'(Name, Arity, _Path).
  195
  196op_type(X, prefix) :-
  197    atom_chars(X, [f, _]).
  198op_type(X, infix) :-
  199    atom_chars(X, [_, f, _]).
  200op_type(X, postfix) :-
  201    atom_chars(X, [_, f]).
  202
  203printable(C) :-
  204    integer(C),
  205    code_type(C, graph).
  206
  207
  208                /********************************
  209                *             ATOMS             *
  210                *********************************/
  211
  212explain_atom(A, Explanation) :-
  213    referenced(A, Explanation).
  214explain_atom(A, Explanation) :-
  215    current_predicate(A, Module:Head),
  216    (   Module == system
  217    ->  true
  218    ;   \+ predicate_property(Module:Head, imported_from(_))
  219    ),
  220    explain_predicate(Module:Head, Explanation).
  221explain_atom(A, Explanation) :-
  222    predicate_property(Module:Head, undefined),
  223    functor(Head, A, _),
  224    explain_predicate(Module:Head, Explanation).
  225
  226
  227                /********************************
  228                *            FUNCTOR             *
  229                *********************************/
  230
  231explain_functor(Head, Explanation) :-
  232    referenced(Head, Explanation).
  233explain_functor(Head, Explanation) :-
  234    current_predicate(_, Module:Head),
  235    \+ predicate_property(Module:Head, imported_from(_)),
  236    explain_predicate(Module:Head, Explanation).
  237explain_functor(Head, Explanation) :-
  238    predicate_property(M:Head, undefined),
  239    (   functor(Head, N, A),
  240        Explanation = [ pi(M:N/A), 'is an undefined predicate' ]
  241    ;   referenced(M:Head, Explanation)
  242    ).
  243
  244
  245                /********************************
  246                *           PREDICATE           *
  247                *********************************/
  248
  249lproperty(built_in,     [' built-in']).
  250lproperty(dynamic,      [' dynamic']).
  251lproperty(multifile,    [' multifile']).
  252lproperty(transparent,  [' meta']).
  253
  254tproperty(Pred, [' imported from module ', module(Module)]) :-
  255    predicate_property(Pred, imported(Module)).
  256tproperty(Pred, [' defined in ', url(File:Line)]) :-
  257    predicate_property(Pred, file(File)),
  258    predicate_property(Pred, line_count(Line)).
  259tproperty(Pred, [' that can be autoloaded']) :-
  260    predicate_property(Pred, autoload).
 explain_predicate(:Head, -Explanation) is det
  264explain_predicate(Pred, Explanation) :-
  265    Pred = Module:Head,
  266    functor(Head, Name, Arity),
  267    (   predicate_property(Pred, non_terminal)
  268    ->  What = 'non-terminal'
  269    ;   What = 'predicate'
  270    ),
  271    (   predicate_property(Pred, undefined)
  272    ->  Explanation = [ pi(Module:Name/Arity),
  273                        ansi([bold,fg(default)], ' is an undefined ~w', [What])
  274                      ]
  275    ;   (   var(Module)
  276        ->  U0 = [ pi(Name/Arity),
  277                   ansi([bold,fg(default)], ' is a', [])
  278                 ]
  279        ;   U0 = [ pi(Module:Name/Arity),
  280                   ansi([bold,fg(default)], ' is a', [])
  281                 ]
  282        ),
  283        findall(Utter, (lproperty(Prop, Utter),
  284                        predicate_property(Pred, Prop)),
  285                U1),
  286        U2 = [ansi([bold,fg(default)], ' ~w', [What]) ],
  287        findall(Utter, tproperty(Pred, Utter),
  288                U3),
  289        flatten([U0, U1, U2, U3], Explanation)
  290    ).
  291:- if(current_predicate(man_object_property/2)).  292explain_predicate(Pred, Explanation) :-
  293    Pred = _Module:Head,
  294    functor(Head, Name, Arity),
  295    man_object_property(Name/Arity, summary(Summary)),
  296    source_file(Pred, File),
  297    current_prolog_flag(home, Home),
  298    sub_atom(File, 0, _, _, Home),
  299    Explanation = [indent, 'Summary: "~w"'-[Summary] ].
  300:- endif.  301explain_predicate(Pred, Explanation) :-
  302    referenced(Pred, Explanation).
  303
  304                /********************************
  305                *          REFERENCES           *
  306                *********************************/
  307
  308referenced(Term, Explanation) :-
  309    current_predicate(_, Module:Head),
  310    (   predicate_property(Module:Head, built_in)
  311    ->  current_prolog_flag(access_level, system)
  312    ;   true
  313    ),
  314    \+ predicate_property(Module:Head, imported_from(_)),
  315    Module:Head \= help_index:predicate(_,_,_,_,_),
  316    nth_clause(Module:Head, N, Ref),
  317    '$xr_member'(Ref, Term),
  318    utter_referenced(Module:Head, N, Ref,
  319                     'Referenced', Explanation).
  320referenced(_:Head, Explanation) :-
  321    current_predicate(_, Module:Head),
  322    (   predicate_property(Module:Head, built_in)
  323    ->  current_prolog_flag(access_level, system)
  324    ;   true
  325    ),
  326    \+ predicate_property(Module:Head, imported_from(_)),
  327    nth_clause(Module:Head, N, Ref),
  328    '$xr_member'(Ref, Head),
  329    utter_referenced(Module:Head, N, Ref,
  330                     'Possibly referenced', Explanation).
  331
  332utter_referenced(_Module:class(_,_,_,_,_,_), _, _, _, _) :-
  333    current_prolog_flag(xpce, true),
  334    !,
  335    fail.
  336utter_referenced(_Module:lazy_send_method(_,_,_), _, _, _, _) :-
  337    current_prolog_flag(xpce, true),
  338    !,
  339    fail.
  340utter_referenced(_Module:lazy_get_method(_,_,_), _, _, _, _) :-
  341    current_prolog_flag(xpce, true),
  342    !,
  343    fail.
  344utter_referenced(From, _, _, _, _) :-
  345    hide_reference(From),
  346    !,
  347    fail.
  348utter_referenced(pce_xref:defined(_,_,_), _, _, _, _) :-
  349    !,
  350    fail.
  351utter_referenced(pce_xref:called(_,_,_), _, _, _, _) :-
  352    !,
  353    fail.
  354utter_referenced(pce_principal:send_implementation(_, _, _),
  355                 _, Ref, Text, Explanation) :-
  356    current_prolog_flag(xpce, true),
  357    !,
  358    xpce_method_id(Ref, Id),
  359    Explanation = [indent, '~w from ~w'-[Text, Id]].
  360utter_referenced(pce_principal:get_implementation(Id, _, _, _),
  361                 _, Ref, Text, Explanation) :-
  362    current_prolog_flag(xpce, true),
  363    !,
  364    xpce_method_id(Ref, Id),
  365    Explanation = [indent, '~w from ~w'-[Text, Id]].
  366utter_referenced(Head, N, Ref, Text, Explanation) :-
  367    clause_property(Ref, file(File)),
  368    clause_property(Ref, line_count(Line)),
  369    !,
  370    pi_head(PI, Head),
  371    Explanation = [ indent,
  372                    '~w from ~d-th clause of '-[Text, N],
  373                    pi(PI), ' at ', url(File:Line)
  374                  ].
  375utter_referenced(Head, N, _Ref, Text, Explanation) :-
  376    pi_head(PI, Head),
  377    Explanation = [ indent,
  378                    '~w from ~d-th clause of '-[Text, N],
  379                    pi(PI)
  380                  ].
  381
  382xpce_method_id(Ref, Id) :-
  383    clause(Head, _Body, Ref),
  384    strip_module(Head, _, H),
  385    arg(1, H, Id).
  386
  387hide_reference(pce_xref:exported(_,_)).
  388hide_reference(pce_xref:defined(_,_,_)).
  389hide_reference(pce_xref:called(_,_,_)).
  390hide_reference(prolog_xref:called(_,_,_,_,_)).
  391hide_reference(prolog_xref:pred_mode(_,_,_)).
  392hide_reference(prolog_xref:exported(_,_)).
  393hide_reference(prolog_xref:dynamic(_,_,_)).
  394hide_reference(prolog_xref:imported(_,_,_)).
  395hide_reference(prolog_xref:pred_comment(_,_,_,_)).
  396hide_reference(_:'$mode'(_,_)).
  397hide_reference(_:'$pldoc'(_,_,_,_)).
  398hide_reference(prolog_manual_index:man_index(_,_,_,_,_)).
  399
  400
  401                /********************************
  402                *           MESSAGES            *
  403                *********************************/
  404
  405:- multifile
  406    prolog:message//1.  407
  408prolog:message(explain(Explanation)) -->
  409    report(Explanation).
  410
  411report(Explanation) -->
  412    { string(Explanation),
  413      !,
  414      split_string(Explanation, "\n", "", Lines)
  415    },
  416    lines(Lines).
  417report(Explanation) -->
  418    { is_list(Explanation) },
  419    report_list(Explanation).
  420
  421lines([]) -->
  422    [].
  423lines([H]) -->
  424    !,
  425    [ '~s'-[H] ].
  426lines([H|T]) -->
  427    [ '~s'-[H], nl ],
  428    lines(T).
  429
  430report_list([]) -->
  431    [].
  432report_list([H|T]) -->
  433    report1(H),
  434    report_list(T).
  435
  436report1(indent) -->
  437    !,
  438    [ '~t~6|'-[] ].
  439report1(String) -->
  440    { atomic(String) },
  441    [ '~w'-[String] ].
  442report1(Fmt-Args) -->
  443    !,
  444    [ Fmt-Args ].
  445report1(url(Location)) -->
  446    [ url(Location) ].
  447report1(url(URL, Label)) -->
  448    [ url(URL, Label) ].
  449report1(pi(PI)) -->
  450    { pi_nt(PI, NT) },
  451    [ ansi(code, '~q', [NT]) ].
  452report1(ansi(Style, Fmt, Args)) -->
  453    [ ansi(Style, Fmt, Args) ].
  454report1(isa(Obj, Fmt-Args)) -->
  455    !,
  456    [ ansi(code, '~p', [Obj]),
  457      ansi([bold,fg(default)], ' is ', []),
  458      ansi([bold,fg(default)], Fmt, Args)
  459    ].
  460report1(isa(Obj, Descr)) -->
  461    [ ansi(code, '~p', [Obj]),
  462      ansi([bold,fg(default)], ' is ~w', [Descr])
  463    ].
  464
  465pi_nt(Module:Name/Arity, NT),
  466    atom(Module), atom(Name), integer(Arity),
  467    Arity >= 2,
  468    functor(Head, Name, Arity),
  469    predicate_property(Module:Head, non_terminal) =>
  470    Arity2 is Arity - 2,
  471    NT = Module:Name//Arity2.
  472pi_nt(PI, NT) =>
  473    NT = PI