135 lines
3.7 KiB
Prolog
135 lines
3.7 KiB
Prolog
/*************************************************************************
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* *
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* YAP Prolog *
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* *
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* Yap Prolog was developed at NCCUP - Universidade do Porto *
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* *
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* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 *
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* *
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**************************************************************************
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* *
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* File: undefined.yap *
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* Last rev: 8/2/88 *
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* mods: *
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* comments: Predicate Undefined for YAP *
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* *
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*************************************************************************/
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/** @defgroup Undefined_Procedures Handling Undefined Procedures
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@ingroup YAPControl
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@{
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A predicate in a module is said to be undefined if there are no clauses
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defining the predicate, and if the predicate has not been declared to be
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dynamic. What YAP does when trying to execute undefined predicates can
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be specified in three different ways:
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+ By setting an YAP flag, through the yap_flag/2 or
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set_prolog_flag/2 built-ins. This solution generalizes the
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ISO standard by allowing module-specific behavior.
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+ By using the unknown/2 built-in (this deprecated solution is
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compatible with previous releases of YAP).
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+ By defining clauses for the hook predicate
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`user:unknown_predicate_handler/3`. This solution is compatible
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with SICStus Prolog.
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*/
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'$undefp'([M0|G0], Default) :-
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% make sure we do not loop on undefined predicates
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% for undefined_predicates.
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'$enter_undefp',
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(
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'$get_undefined_pred'(G0, M0, Goal, NM)
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->
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'$exit_undefp',
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Goal \= fail,
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'$complete_goal'(M0, G0, Goal, NM, NG),
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'$execute0'(NG, NM)
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;
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user:unknown_predicate_handler(G0,M0,NG)
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->
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'$exit_undefp',
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call(M0:NG)
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;
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'$messages' = M0,
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fail
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;
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'$exit_undefp',
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'$handle_error'(Default,G0,M0)
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).
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/** @pred unknown(- _O_,+ _N_)
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The unknown predicate, informs about what the user wants to be done
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when there are no clauses for a certain predicate.
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This predicate is strongly deprecated. Use prolog_flag for generic
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behaviour, and user:unknown_predicate_handler/3 for flexible behaviour
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on undefined goals.
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*/
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unknown(P, NP) :-
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prolog_flag( unknown, P, NP ).
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/** @pred user:unknown_predicate_handler(+ _Call_, + _M_, - _N_)
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In YAP, the default action on undefined predicates is to output an
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`error` message. Alternatives are to silently `fail`, or to print a
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`warning` message and then fail. This follows the ISO Prolog standard
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where the default action is `error`.
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The user:unknown_predicate_handler/3 hook was originally include in
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SICStus Prolog. It allows redefining the answer for specifici
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calls. As an example. after defining `undefined/1` by:
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~~~~~{.prolog}
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undefined(A) :- format('Undefined predicate: ~w~n',[A]), fail.
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~~~~~
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and executing the goal:
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~~~~~{.prolog}
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:- assert(user:unknown_predicate_handler(U,M,undefined(M:U)) )
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~~~~~
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a call to a predicate for which no clauses were defined will result in
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the output of a message of the form:
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~~~~~{.prolog}
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Undefined predicate: user:xyz(A1,A2)
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~~~~~
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followed by the failure of that call.
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*/
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:- multifile user:unknown_predicate_handler/3.
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'$handle_error'(error,Goal,Mod) :-
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functor(Goal,Name,Arity),
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'$program_continuation'(PMod,PName,PAr),
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'$do_error'(existence_error(procedure,Name/Arity),context(Mod:Goal,PMod:PName/PAr)).
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'$handle_error'(warning,Goal,Mod) :-
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functor(Goal,Name,Arity),
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'$program_continuation'(PMod,PName,PAr),
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print_message(warning,error(existence_error(procedure,Name/Arity), context(Mod:Goal,PMod:PName/PAr))),
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fail.
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'$handle_error'(fail,_Goal,_Mod) :-
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fail.
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'$complete_goal'(M, G, CurG, CurMod, NG) :-
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(
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'$is_metapredicate'(CurG,CurMod)
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->
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'$meta_expansion'(G, M, CurMod, M, NG, [])
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;
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NG = G
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).
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/**
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@}
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*/
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