yap-6.3/pl/consult.yap

/*************************************************************************
*									 *
*	 YAP Prolog 							 *
*									 *
*	Yap Prolog was developed at NCCUP - Universidade do Porto	 *
*									 *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997	 *
*									 *
**************************************************************************
*									 *
* File:		consult.yap						 *
* Last rev:	8/2/88							 *
* mods:									 *
* comments:	Consulting Files in YAP					 *
*									 *
*************************************************************************/

%
% SWI options
% autoload(true,false)
% derived_from(File) -> make
% encoding(Encoding) => implemented
% expand({true,false)
% if(changed,true,not_loaded) => implemented
% imports(all,List) => implemented
% qcompile(true,false)
% silent(true,false)  => implemented
% stream(Stream)  => implemented
% consult(consult,reconsult) => implemented
% compilation_mode(compile,source,assert_all) => implemented
%
load_files(Files,Opts) :-
	'$load_files'(Files,Opts,load_files(Files,Opts)).

'$load_files'(Files,Opts,Call) :-		    
	'$process_lf_opts'(Opts,Silent,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,Files,Call),
	'$check_use_module'(Call,UseModule),
        '$current_module'(M0),
	'$lf'(Files,M0,Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,UseModule),
	'$close_lf'(Silent).

'$process_lf_opts'(V,_,_,_,_,_,_,_,_,_,_,_,_,Call) :-
	var(V), !,
	'$do_error'(instantiation_error,Call).
'$process_lf_opts'([],_,_,_,_,_,_,_,Encoding,_,_,_,_,_) :-
	(var(Encoding) ->
	    '$default_encoding'(Encoding)
	    ;
	    true
	).
'$process_lf_opts'([Opt|Opts],Silent,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,Files,Call) :-
	'$process_lf_opt'(Opt,Silent,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,Files,Call), !, 
	'$process_lf_opts'(Opts,Silent,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,Files,Call).
'$process_lf_opts'([Opt|_],_,_,_,_,_,_,_,_,_,_,_,_,Call) :-
	'$do_error'(domain_error(unimplemented_option,Opt),Call).

'$process_lf_opt'(autoload(true),_,InfLevel,_,_,_,_,_,_,_,_,_,_,_) :-
	get_value('$verbose_auto_load',VAL),
	(VAL = true ->
	    InfLevel = informational
	;
	    InfLevel = silent
	).
'$process_lf_opt'(autoload(false),_,_,_,_,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(derived_from(File),_,_,_,_,_,_,_,_,_,_,_,Files,Call) :-
	( atom(File) -> true ;  '$do_error'(type_error(atom,File),Call) ),
	( atom(Files) -> true ;  '$do_error'(type_error(atom,Files),Call) ),
	/* call make */
	'$do_error'(domain_error(unimplemented_option,derived_from),Call).
'$process_lf_opt'(encoding(Encoding),_,_,_,_,_,_,_,_,EncCode,_,_,_,Call) :-
	( var(Encoding) ->
	    '$do_error'(instantiation_error,Call)
	;
	    '$valid_encoding'(Encoding, EncCode) ->
	    true
	;	
	    '$do_error'(domain_error(io_mode,encoding(Encoding)),Call)
	).
'$process_lf_opt'(expand(true),_,_,true,_,_,_,_,_,_,_,_,_,Call) :-
	'$do_error'(domain_error(unimplemented_option,expand),Call).
'$process_lf_opt'(expand(false),_,_,false,_,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(if(changed),_,_,_,changed,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(if(true),_,_,_,true,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(if(not_loaded),_,_,_,not_loaded,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(imports(all),_,_,_,_,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(imports(Imports),_,_,_,_,_,Imports,_,_,_,_,_,_,_).
'$process_lf_opt'(qcompile(true),_,_,_,_,true,_,_,_,_,_,_,_,Call) :-
	'$do_error'(domain_error(unimplemented_option,qcompile),Call).
'$process_lf_opt'(qcompile(false),_,_,_,_,false,_,_,_,_,_,_,_,_).
'$process_lf_opt'(silent(true),Silent,silent,_,_,_,_,_,_,_,_,_,_,_) :-
	( nb_getval('$lf_verbose',Silent) -> true ;  Silent = informational),
	nb_setval('$lf_verbose',silent).
'$process_lf_opt'(skip_unix_comments,_,_,_,_,_,_,_,_,skip_unix_comments,_,_,_,_).
'$process_lf_opt'(compilation_mode(source),_,_,_,_,_,_,_,_,_,source,_,_,_).
'$process_lf_opt'(compilation_mode(compile),_,_,_,_,_,_,_,_,_,compile,_,_,_).
'$process_lf_opt'(compilation_mode(assert_all),_,_,_,_,_,_,_,_,_,assert_all,_,_,_).
'$process_lf_opt'(silent(false),_,_,_,_,_,_,_,_,_,_,_,_,_).
'$process_lf_opt'(consult(reconsult),_,_,_,_,_,_,_,_,_,_,reconsult,_,_).
'$process_lf_opt'(consult(consult),_,_,_,_,_,_,_,_,_,_,consult,_,_).
'$process_lf_opt'(stream(Stream),_,_,_,_,_,_,Stream,_,_,_,_,Files,Call) :-
/*	( '$stream'(Stream) -> true ;  '$do_error'(domain_error(stream,Stream),Call) ), */
	( atom(Files) -> true ;  '$do_error'(type_error(atom,Files),Call) ).

'$check_use_module'(use_module(_),use_module(_)) :- !.
'$check_use_module'(use_module(_,_),use_module(_)) :- !.
'$check_use_module'(use_module(M,_,_),use_module(M)) :- !.
'$check_use_module'(_,load_files) :- !.

'$lf'(V,_,Call,_,_,_,_,_,_,_,_,_,_) :- var(V), !,
	'$do_error'(instantiation_error,Call).
'$lf'([],_,_,_,_,_,_,_,_,_,_,_,_,_) :- !.
'$lf'(M:X, _, Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,UseModule) :- !,
	(
	  atom(M)
	->
	  '$lf'(X, M, Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,UseModule)
	  ;
	  '$do_error'(type_error(atom,M),Call)
	).
'$lf'([F|Fs], Mod,Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,UseModule) :- !,
	'$lf'(F,Mod,Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,_),
	'$lf'(Fs, Mod,Call,InfLevel,Expand,Changed,CompilationMode,Imports,Stream,Encoding,SkipUnixComments,CompMode,Reconsult,UseModule).
'$lf'(_, Mod, _,InfLevel,_,_,CompilationMode,Imports,Stream,_,Reconsult,SkipUnixComments,CompMode,UseModule) :-
        nonvar(Stream), !,
	'$do_lf'(Mod, Stream, InfLevel,CompilationMode,Imports,SkipUnixComments,CompMode,Reconsult,UseModule).
'$lf'(user, Mod, _,InfLevel,_,_,CompilationMode,Imports,_,_,SkipUnixComments,CompMode,Reconsult,UseModule) :- !,
	'$do_lf'(Mod, user_input, InfLevel, CompilationMode,Imports,SkipUnixComments,CompMode,Reconsult,UseModule).
'$lf'(user_input, Mod, _,InfLevel,_,_,CompilationMode,Imports,_,_,SkipUnixComments,CompMode,Reconsult,UseModule) :- !,
	'$do_lf'(Mod, user_input, InfLevel, CompilationMode,Imports,SkipUnixComments,CompMode,Reconsult,UseModule).
'$lf'(X, Mod, Call, InfLevel,_,Changed,CompilationMode,Imports,_,Enc,SkipUnixComments,CompMode,Reconsult,UseModule) :-
	'$find_in_path'(X, Y, Call),
	'$open'(Y, '$csult', Stream, 0, Enc), !,
	'$set_changed_lfmode'(Changed),
	'$start_lf'(X, Mod, Stream, InfLevel, CompilationMode, Imports, Changed,SkipUnixComments,CompMode,Reconsult,UseModule),
	'$close'(Stream).
'$lf'(X, _, Call, _, _, _, _, _, _, _, _, _, _, _) :-
	'$do_error'(permission_error(input,stream,X),Call).

'$set_changed_lfmode'(true) :- !.
'$set_changed_lfmode'(_).

'$start_lf'(_, Mod, Stream,_ ,_, Imports, not_loaded, _, _, _, _) :-
	'$file_loaded'(Stream, Mod, Imports), !.
'$start_lf'(_, Mod, Stream, _, _, Imports, changed, _, _, _, _) :-
	'$file_unchanged'(Stream, Mod, Imports), !.
'$start_lf'(_, Mod, Stream, InfLevel, CompilationMode, Imports, _, SkipUnixComments, CompMode, Reconsult, UseModule) :-
	'$do_lf'(Mod, Stream, InfLevel, CompilationMode, Imports, SkipUnixComments, CompMode, Reconsult, UseModule).

'$close_lf'(Silent) :- 
	nonvar(Silent), !,
	nb_setval('$lf_verbose',Silent).
'$close_lf'(_).

ensure_loaded(Fs) :-
	'$load_files'(Fs, [if(changed)],ensure_loaded(Fs)).

compile(Fs) :-
	'$load_files'(Fs, [], compile(Fs)).

consult(Fs) :-
	'$has_yap_or',
	'$do_error'(context_error(consult(Fs),clause),query).
consult(V) :-
	var(V), !,
	'$do_error'(instantiation_error,consult(V)).
consult(M0:Fs) :- !,
	'$consult'(Fs, M0).
consult(Fs) :-
	'$current_module'(M0),
	'$consult'(Fs, M0).

'$consult'(Fs,Module) :-
	'$access_yap_flags'(8, 2), % SICStus Prolog compatibility
	!,
	'$load_files'(Module:Fs,[],Fs).
'$consult'(Fs, Module) :-
	'$load_files'(Module:Fs,[consult(consult)],Fs).

reconsult(Fs) :-
	'$load_files'(Fs, [], reconsult(Fs)).

use_module(F) :-
	'$load_files'(F, [if(not_loaded)], use_module(F)).

use_module(F,Is) :-
	'$load_files'(F, [if(not_loaded),imports(Is)], use_module(F,Is)).

use_module(M,F,Is) :-
	'$use_module'(M,F,Is).

'$use_module'(U,_F,Is) :- nonvar(U), U = user, !,
	'$import_to_current_module'(user_input, user, Is).
'$use_module'(M,F,Is) :- nonvar(M), !,
	recorded('$module','$module'(F1,M,_),_),
	'$load_files'(F1, [if(not_loaded),imports(Is)], use_module(M,F,Is)),
	F1 = F.
'$use_module'(M,F,Is) :- nonvar(F),
	'$load_files'(F, [if(not_loaded),imports(Is)], use_module(M,F,Is)).

'$csult'(V, _) :- var(V), !,
	'$do_error'(instantiation_error,consult(V)).
'$csult'([], _).
'$csult'([-F|L], M) :- !, '$load_files'(M:F, [],[-M:F]), '$csult'(L, M).
'$csult'([F|L], M) :- '$consult'(F, M), '$csult'(L, M).

'$do_lf'(ContextModule, Stream, InfLevel, _, Imports, SkipUnixComments, CompMode, Reconsult, UseModule) :-
	nb_getval('$if_level',OldIncludeLevel),
	nb_setval('$if_level',0),
	nb_getval('$system_mode', OldMode),
        ( OldMode == off -> '$enter_system_mode' ; true ),
	'$record_loaded'(Stream, ContextModule),
	'$current_module'(OldModule,ContextModule),
	getcwd(OldD),
	nb_getval('$consulting_file',OldF),
	'$set_consulting_file'(Stream),
	H0 is heapused, '$cputime'(T0,_),
	'$current_stream'(File,_,Stream),
	'$fetch_stream_alias'(OldStream,'$loop_stream'),
	'$change_alias_to_stream'('$loop_stream',Stream),
	nb_getval('$consulting',Old),
	nb_setval('$consulting',false),
	'$access_yap_flags'(18,GenerateDebug),
	'$consult_infolevel'(InfLevel),
	'$comp_mode'(OldCompMode, CompMode),
	recorda('$initialisation','$',_),
	( Reconsult = reconsult ->
	    '$start_reconsulting'(File),
	    '$start_consult'(Reconsult,File,LC),
	    '$remove_multifile_clauses'(File),
	    StartMsg = reconsulting,
	    EndMsg = reconsulted
	    ;
	    '$start_consult'(Reconsult,File,LC),
	    StartMsg = consulting,
	    EndMsg = consulted
	),
	print_message(InfLevel, loading(StartMsg, File)),
	( SkipUnixComments == skip_unix_comments ->
	    '$skip_unix_comments'(Stream)
	;
	    true
	),
	'$loop'(Stream,Reconsult),
	H is heapused-H0, '$cputime'(TF,_), T is TF-T0,
	'$current_module'(Mod,OldModule),
	print_message(InfLevel, loaded(EndMsg, File, Mod, T, H)),
	'$end_consult',
	( 
	    Reconsult = reconsult ->
	    '$clear_reconsulting'
	;
	    true
	),
	'$change_alias_to_stream'('$loop_stream',OldStream),
	'$set_yap_flags'(18,GenerateDebug),
	'$comp_mode'(_, OldCompMode),
	nb_setval('$consulting',Old),
	nb_setval('$consulting_file',OldF),
	cd(OldD),
	nb_setval('$if_level',OldIncludeLevel),
	% surely, we were in run mode or we would not have included the file!
	nb_setval('$if_skip_mode',run),
	% back to include mode!
	nb_setval('$if_level',OldIncludeLevel),
	'$bind_module'(Mod, UseModule),
	'$import_to_current_module'(File, ContextModule, Imports),
	( LC == 0 -> prompt(_,'   |: ') ; true),
        ( OldMode == off -> '$exit_system_mode' ; true ),
	'$exec_initialisation_goals',
	!.

'$bind_module'(_, load_files).
'$bind_module'(Mod, use_module(Mod)).

'$import_to_current_module'(File, M, Imports) :-
	recorded('$module','$module'(File,NM,Ps),_), M \= NM, !,
	'$use_preds'(Imports, Ps, NM, M).
'$import_to_current_module'(_, _, _).

'$consult_infolevel'(InfoLevel) :- nonvar(InfoLevel), !.
'$consult_infolevel'(InfoLevel) :-
	nb_getval('$lf_verbose',InfoLevel), InfoLevel \= [], !.
'$consult_infolevel'(informational).

'$start_reconsulting'(F) :-
	recorda('$reconsulted','$',_),
	recorda('$reconsulting',F,_).

'$initialization'(V) :-
	var(V), !,
	'$do_error'(instantiation_error,initialization(V)).
'$initialization'(C) :- number(C), !,
	'$do_error'(type_error(callable,C),initialization(C)).
'$initialization'(C) :- db_reference(C), !,
	'$do_error'(type_error(callable,C),initialization(C)).
'$initialization'(G) :-
	'$show_consult_level'(Level1),
	% it will be done after we leave the current consult level.
	Level is Level1-1,
	recorda('$initialisation',do(Level,G),_),
	fail.
'$initialization'(_).

'$exec_initialisation_goals' :-
	recorded('$blocking_code',_,R),
	erase(R),
	fail.
% system goals must be performed first 
'$exec_initialisation_goals' :-
	recorded('$system_initialisation',G,R),
	erase(R),
	G \= '$',
	call(G),
	fail.
'$exec_initialisation_goals' :-
	'$show_consult_level'(Level),
	recorded('$initialisation',do(Level,G),R),
	erase(R),
	G \= '$',
	'$current_module'(M),
	nb_getval('$system_mode', OldMode),
        ( OldMode == on -> '$exit_system_mode' ; true ),
	% run initialization under user control (so allow debugging this stuff).
	(
	  '$system_catch'(once(M:G), M, Error, user:'$LoopError'(Error, top)),
	  fail
	;
          OldMode = on,
	  '$enter_system_mode',
	  fail
	).
'$exec_initialisation_goals'.

'$include'(V, _) :- var(V), !,
	'$do_error'(instantiation_error,include(V)).
'$include'([], _) :- !.
'$include'([F|Fs], Status) :- !,
	'$include'(F, Status),
	'$include'(Fs, Status).
'$include'(X, Status) :-
	nb_getval('$lf_verbose',Verbosity),
	'$find_in_path'(X,Y,include(X)),
	nb_getval('$included_file',OY),
	nb_setval('$included_file', Y),
	'$current_module'(Mod),
	H0 is heapused, '$cputime'(T0,_),
	'$default_encoding'(Encoding),
	( '$open'(Y,'$csult',Stream,0,Encoding), !,
		print_message(Verbosity, loading(including, Y)),
		'$loop'(Stream,Status), '$close'(Stream)
	;
		'$do_error'(permission_error(input,stream,Y),include(X))
	),
	H is heapused-H0, '$cputime'(TF,_), T is TF-T0,
	print_message(Verbosity, loaded(included, Y, Mod, T, H)),
	nb_setval('$included_file',OY).

'$do_startup_reconsult'(X) :-
	( '$access_yap_flags'(15, 0) ->
	  '$system_catch'(load_files(X, [silent(true)]),Module,Error,'$Error'(Error))
	;
	  set_value('$verbose',off),
	  '$system_catch'(load_files(X, [silent(true),skip_unix_comments]),Module,_,fail)
	;
	  true
	),
	!,
	( '$access_yap_flags'(15, 0) -> true ; halt).

'$skip_unix_comments'(Stream) :-
	'$peek'(Stream, 0'#), !, % 35 is ASCII for #
	'$get0_line_codes'(Stream, _),
	'$skip_unix_comments'(Stream).
'$skip_unix_comments'(_).


prolog_load_context(_, _) :-
	nb_getval('$consulting_file',[]), !, fail.
prolog_load_context(directory, DirName) :- 
	getcwd(DirName).
prolog_load_context(file, FileName) :- 
	nb_getval('$included_file',IncFileName),
	( IncFileName = [] ->
	  nb_getval('$consulting_file',FileName)
        ;
           FileName  = IncFileName
        ).
prolog_load_context(module, X) :-
	'$current_module'(X).
prolog_load_context(source, FileName) :-
	nb_getval('$consulting_file',FileName).
prolog_load_context(stream, Stream) :- 
	'$fetch_stream_alias'(Stream,'$loop_stream').
prolog_load_context(term_position, Position) :- 
	'$fetch_stream_alias'(Stream,'$loop_stream'),
	stream_position(Stream, Position).


% if the file exports a module, then we can
% be imported from any module.
'$file_loaded'(Stream, M, Imports) :-
	'$file_name'(Stream, F),
	'$ensure_file_loaded'(F, M, Imports).

'$ensure_file_loaded'(F, M, Imports) :-
	recorded('$module','$module'(F1,NM,P),_),
	recorded('$lf_loaded','$lf_loaded'(F1,_,_),_),
	'$same_file'(F1,F), !,
	'$use_preds'(Imports,P, NM, M).
'$ensure_file_loaded'(F, M, _) :-
	recorded('$lf_loaded','$lf_loaded'(F1,M,_),_),
	'$same_file'(F1,F), !.
	

% if the file exports a module, then we can
% be imported from any module.
'$file_unchanged'(Stream, M, Imports) :-
	'$file_name'(Stream, F),
	'$ensure_file_unchanged'(F, M, Imports).

'$ensure_file_unchanged'(F, M, Imports) :-
	recorded('$module','$module'(F1,NM,P),_),
	recorded('$lf_loaded','$lf_loaded'(F1,_,Age),R),
	'$same_file'(F1,F), !,
	'$file_is_unchanged'(F, R, Age),
	'$use_preds'(Imports, P, NM, M).
'$ensure_file_unchanged'(F, M, _) :-
	recorded('$lf_loaded','$lf_loaded'(F1,M,Age),R),
	'$same_file'(F1,F), !,
	'$file_is_unchanged'(F, R, Age).

'$file_is_unchanged'(F, R, Age) :-
        '$file_age'(F,CurrentAge),
         ((CurrentAge = Age ; Age = -1)  -> true; erase(R), fail).



path(Path) :- findall(X,'$in_path'(X),Path).

'$in_path'(X) :- recorded('$path',Path,_),
		atom_codes(Path,S),
		( S = ""  -> X = '.' ;
		  atom_codes(X,S) ).

add_to_path(New) :- add_to_path(New,last).

add_to_path(New,Pos) :-
	atom(New), !,
	'$check_path'(New,Str),
	atom_codes(Path,Str),
	'$add_to_path'(Path,Pos).

'$add_to_path'(New,_) :- recorded('$path',New,R), erase(R), fail.
'$add_to_path'(New,last) :- !, recordz('$path',New,_).
'$add_to_path'(New,first) :- recorda('$path',New,_).

remove_from_path(New) :- '$check_path'(New,Path),
			recorded('$path',Path,R), erase(R).

'$check_path'(At,SAt) :- atom(At), !, atom_codes(At,S), '$check_path'(S,SAt).
'$check_path'([],[]).
'$check_path'([Ch],[Ch]) :- '$dir_separator'(Ch), !.
'$check_path'([Ch],[Ch,A]) :- !, integer(Ch), '$dir_separator'(A).
'$check_path'([N|S],[N|SN]) :- integer(N), '$check_path'(S,SN).

% add_multifile_predicate when we start consult
'$add_multifile'(Name,Arity,Module) :-
	nb_getval('$consulting_file',File),
	'$add_multifile'(File,Name,Arity,Module).

'$add_multifile'(File,Name,Arity,Module) :-
	recorded('$multifile_defs','$defined'(File,Name,Arity,Module), _), !,
	print_message(warning,declaration((multifile Module:Name/Arity),ignored)).
'$add_multifile'(File,Name,Arity,Module) :-
	recordz('$multifile_defs','$defined'(File,Name,Arity,Module),_), !,
	fail.
'$add_multifile'(File,Name,Arity,Module) :-
	recorded('$mf','$mf_clause'(File,Name,Arity,Module,Ref),R),
	erase(R),
	'$erase_clause'(Ref,Module),
	fail.
'$add_multifile'(_,_,_,_).

% retract old multifile clauses for current file.
'$remove_multifile_clauses'(FileName) :-
	recorded('$multifile_defs','$defined'(FileName,_,_,_),R1),
	erase(R1),
	fail.
'$remove_multifile_clauses'(FileName) :-
	recorded('$mf','$mf_clause'(FileName,_,_,Module,Ref),R),
	'$erase_clause'(Ref, Module),
	erase(R),
	fail.
'$remove_multifile_clauses'(_).

'$set_consulting_file'(user) :- !,
	nb_setval('$consulting_file',user_input).
'$set_consulting_file'(user_input) :- !,
	nb_setval('$consulting_file',user_input).
'$set_consulting_file'(Stream) :-
	'$file_name'(Stream,F),
	nb_setval('$consulting_file',F),
	'$set_consulting_dir'(F).

%
% Use directory where file exists
%
'$set_consulting_dir'(F) :-
	file_directory_name(F, Dir),
	cd(Dir).

'$record_loaded'(Stream, M) :-
	Stream \= user,
	Stream \= user_input,
	'$file_name'(Stream,F),
	( recorded('$lf_loaded','$lf_loaded'(F,M,_),R), erase(R), fail ; true ),

	'$file_age'(F,Age),
	recorda('$lf_loaded','$lf_loaded'(F,M,Age),_),
	fail.
'$record_loaded'(_, _).

%
% encoding stuff: what I believe SWI does.
%
% 8-bit binaries
'$valid_encoding'(octet, 0).
% 7-bit ASCII as America originally intended
'$valid_encoding'(ascii, 2).
% Ye europeaners made it 8 bits
'$valid_encoding'(iso_latin_1, 1).
% UTF-8: default 8 bits but 80 extends to 16bits
'$valid_encoding'(utf8, 8).
% UNICODE: 16 bits throughout, the way it was supposed to be!
'$valid_encoding'(unicode_be, 16).
'$valid_encoding'(unicode_le, 32).
% whatever the system tell us to do.
'$valid_encoding'(text, 4).

'$default_encoding'(DefCode) :- nonvar(DefCode), !,
	'$encoding'('$stream'(0),DefCode),
	'$encoding'('$stream'(1),DefCode),
	'$encoding'('$stream'(2),DefCode),
	set_value('$default_encoding',DefCode).
'$default_encoding'(DefCode) :-
	get_value('$default_encoding',DefCode0),
	( DefCode0 == [] -> 
	    '$get_default_encoding'(DefCode)
	 ;
	    DefCode = DefCode0
	).


'$set_encoding'(V) :- var(V), !,
	'$do_error'(instantiation_error, encoding(V)).
'$set_encoding'(EncAtom) :-
	'$valid_encoding'(EncAtom, Enc), !,
	'$fetch_stream_alias'(Stream,'$loop_stream'),
	'$encoding'(Stream, Enc).
'$set_encoding'(EncAtom) :-
	atom(EncAtom), !,
        '$do_error'(domain_error(encoding,EncAtom),encoding(EncAtom)).
'$set_encoding'(EncAtom) :-
	'$do_error'(type_error(atom,V),encoding(EncAtom)).


absolute_file_name(V,Out) :- var(V), !,
	'$do_error'(instantiation_error, absolute_file_name(V, Out)).
absolute_file_name(user,user) :- !.
absolute_file_name(File0,File) :-
	'$absolute_file_name'(File0,[access(none),file_type(txt),file_errors(fail),solutions(first)],File,absolute_file_name(File0,File)).

'$find_in_path'(F0,F,G) :-
	'$absolute_file_name'(F0,[access(read),file_type(source),file_errors(fail),solutions(first)],F,G).

absolute_file_name(File,TrueFileName,Opts) :-
	var(TrueFileName), !,
	absolute_file_name(File,Opts,TrueFileName).
absolute_file_name(File,Opts,TrueFileName) :-
	'$absolute_file_name'(File,Opts,TrueFileName,absolute_file_name(File,Opts,TrueFileName)).
	
'$absolute_file_name'(File,Opts,TrueFileName,G) :- var(File), !,
	'$do_error'(instantiation_error, G).
'$absolute_file_name'(File,Opts,TrueFileName, G) :-
	'$process_fn_opts'(Opts,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G),
	FoundOne = a(false),
	(
	 '$find_in_path'(File,opts(Extensions,RelTo,Type,Access,FErrors,Expand,Debug),TrueFileName,G),
	  (Solutions = first -> ! ; true),
	   nb_setarg(1, FoundOne, true)
	;

	    FErrors = error, FoundOne = a(false) ->
	   '$do_error'(existence_error(file,File),G)
	 ).
	 

'$process_fn_opts'(V,_,_,_,_,_,_,_,_,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$process_fn_opts'([],[],_,source,read,error,first,false,false,_) :- !.
'$process_fn_opts'([Opt|Opts],Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$process_fn_opt'(Opt,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions0,RelTo0,Type0,Access0,FErrors0,Solutions0,Expand0,Debug0,G),
	'$process_fn_opts'(Opts,Extensions0,RelTo0,Type0,Access0,FErrors0,Solutions0,Expand0,Debug0,G).
'$process_fn_opts'(Opts,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$do_error'(type_error(list,T),G).

'$process_fn_opt'(Opt,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G) :- var(Opt), !,
	'$do_error'(instantiation_error, G).
'$process_fn_opt'(extensions(Extensions),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,_,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$check_fn_extensions'(L,G).
'$process_fn_opt'(relative_to(RelTo),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,_,Type,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$check_atom'(RelTo,G).
'$process_fn_opt'(access(Access),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,_,FErrors,Solutions,Expand,Debug,G) :- !,
	'$check_atom'(Access,G).
'$process_fn_opt'(file_type(Type),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,_,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$check_fn_type'(Type,G).
'$process_fn_opt'(file_errors(FErrors),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,_,Solutions,Expand,Debug,G) :- !,
	'$check_fn_errors'(FErrors,G).
'$process_fn_opt'(solutions(Solutions),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,FErrors,_,Expand,Debug,G) :- !,
	'$check_fn_solutions'(Solutions,G).
'$process_fn_opt'(expand(Expand),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,FErrors,Solutions,_,Debug,G) :- !,
	'$check_true_false'(Expand,G).
'$process_fn_opt'(verbose_file_search(Debug),Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,_,G) :- !,
	'$check_true_false'(Debug,G).
'$process_fn_opt'(Opt,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,Extensions,RelTo,Type,Access,FErrors,Solutions,Expand,Debug,G) :- !,
	'$do_error'(domain_error(file_name_option,T),G).	

'$check_fn_extensions'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_fn_extensions'([],_) :- !.
'$check_fn_extensions'([A|L],G) :- !,
	'$check_atom'(A,G),
	'$check_fn_extensions'(L,G).
'$check_fn_extensions'(T,G) :- !,
	'$do_error'(type_error(list,T),G).

'$check_atom'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_atom'(A,G) :- atom(A), !.
'$check_atom'(T,G) :- !,
	'$do_error'(type_error(atom,T),G).
	
'$check_fn_type'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_fn_type'(txt,_) :- !.
'$check_fn_type'(prolog,_) :- !.
'$check_fn_type'(source,_) :- !.
'$check_fn_type'(executable,_) :- !.
'$check_fn_type'(qlf,_) :- !.
'$check_fn_type'(directory,_) :- !.
'$check_fn_type'(T,G) :- atom(T), !,
	'$do_error'(domain_error(file_type,T),G).
'$check_fn_type'(T,G) :- !,
	'$do_error'(type_error(atom,T),G).
	
'$check_fn_errors'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_fn_errors'(fail,_) :- !.
'$check_fn_errors'(error,_) :- !.
'$check_fn_errors'(T,G) :- atom(T), !,
	'$do_error'(domain_error(file_errors,T),G).
'$check_fn_errors'(T,G) :- !,
	'$do_error'(type_error(atom,T),G).
	
'$check_fn_solutions'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_fn_solutions'(first,_) :- !.
'$check_fn_solutions'(all,_) :- !.
'$check_fn_solutions'(T,G) :- atom(T), !,
	'$do_error'(domain_error(solutions,T),G).
'$check_fn_solutions'(T,G) :- !,
	'$do_error'(type_error(atom,T),G).
	
'$check_true_false'(V,G) :- var(V), !,
	'$do_error'(instantiation_error, G).
'$check_true_false'(true,_) :- !.
'$check_true_false'(false,_) :- !.
'$check_true_false'(T,G) :- atom(T), !,
	'$do_error'(domain_error(boolean,T),G).
'$check_true_false'(T,G) :- !,
	'$do_error'(type_error(atom,T),G).
	
% This sequence must be followed:
% user and user_input are special;
% library(F) must check library_directories
% T(F) must check file_search_path
% all must try search in path
'$find_in_path'(user,_,user_input, _) :- !.
'$find_in_path'(user_input,_,user_input, _) :- !.
'$find_in_path'(library(F),_,_, _) :-
	% make sure library_directory is open.
	\+ clause(user:library_directory(_),_),
	'$system_library_directories'(D),
	assert(user:library_directory(D)),
	fail.
'$find_in_path'(library(File),Opts,NewFile, Call) :- !,
	'$dir_separator'(D),
	atom_codes(A,[D]),
	'$extend_path_directory'(library, A, File, Opts, NewFile, Call).
'$find_in_path'(S, Opts, NewFile, Call) :-
	S =.. [Name,File], !,
	'$dir_separator'(D),
	atom_codes(A,[D]),
	'$extend_path_directory'(Name, A, File, Opts, NewFile, Call).
'$find_in_path'(File,Opts,NewFile,_) :-
	atom(File), !,
	'$add_path'(File,PFile),
	'$get_abs_file'(PFile,Opts,AbsFile),
	'$search_in_path'(AbsFile,Opts,NewFile).
'$find_in_path'(File,_,_,Call) :-
	'$do_error'(domain_error(source_sink,File),Call).

'$get_abs_file'(File,opts(_,D0,_,_,_,_,_),AbsFile) :-
	system:true_file_name(File,D0,AbsFile).

'$search_in_path'(File,opts(Extensions,_,_,Access,_,_,_),F) :-
	'$add_extensions'(Extensions,File,F),
	access_file(F,Access).
'$search_in_path'(File,opts(_,_,Type,Access,_,_,_),F) :-
	'$add_type_extensions'(Type,File,F),
	access_file(F,Access).

'$add_extensions'([Ext|_],File,F) :-
	'$mk_sure_true_ext'(Ext,NExt),
	atom_concat([File,NExt],F).
'$add_extensions'([_|Extensions],File,F) :-
	'$add_extensions'(Extensions,File,F).

'$mk_sure_true_ext'(Ext,NExt) :-
	atom_codes(Ext,[C|L]),
	C \= 0'.,
	!,
	atom_codes(NExt,[0'.,C|L]).
'$mk_sure_true_ext'(Ext,Ext).

'$add_type_extensions'(Type,File,F) :-
	'$type_extension'(Type,Ext),
	atom_concat([File,Ext],F).

'$type_extension'(txt,'').
'$type_extension'(prolog,'.yap').
'$type_extension'(prolog,'.pl').
'$type_extension'(prolog,'').
'$type_extension'(source,'.yap').
'$type_extension'(source,'.pl').
'$type_extension'(source,'').
'$type_extension'(executable,'.so').
'$type_extension'(executable,'').
'$type_extension'(qlf,'.qlf').
'$type_extension'(qlf,'').
'$type_extension'(directory,'').

'$add_path'(File,File).
'$add_path'(File,PFile) :-
	recorded('$path',Path,_),
	atom_concat([Path,File],PFile).

'$system_library_directories'(Dir) :-
	getenv('YAPSHAREDIR', Dir).
'$system_library_directories'(Dir) :-
	get_value(system_library_directory,Dir).


'$extend_path_directory'(Name, D, File, Opts, NewFile, Call) :-
	user:file_search_path(Name, Dir),	
	'$extend_pathd'(Dir, D, File, Opts, NewFile, Call).

'$extend_pathd'(Dir, A, File, Opts, NewFile, Call) :-
	atom(Dir), !,
	atom_concat([Dir,A,File],NFile),
	'$find_in_path'(NFile, Opts, NewFile, Goal), !.
'$extend_pathd'(Name, A, File, Opts, OFile, Goal) :-
	nonvar(Name),
	Name =.. [N,P0],
	atom_concat([P0,A,File],NFile),
	NewName =.. [N,NFile],
	'$find_in_path'(NewName, Opts, OFile, Goal).

%
% This is complicated because of embedded ifs.
%
'$if'(_,top) :- !, fail.
'$if'(Goal,_) :-
	nb_getval('$if_level',Level0),
	Level is Level0 + 1,
	nb_setval('$if_level',Level),
	nb_getval('$endif',OldEndif),
	nb_getval('$if_skip_mode',Mode),
	nb_setval('$endif',elif(Level,OldEndif,Mode)),
	fail.
% we are in skip mode, ignore....
'$if'(Goal,_) :-
	nb_getval('$endif',elif(Level, OldEndif, skip)), !,
	nb_setval('$endif',endif(Level, OldEndif, skip)).	
% we are in non skip mode, check....
'$if'(Goal,_) :-
	('$if_call'(Goal)
	    ->
	 % we will execute this branch, and later enter skip
	 nb_getval('$endif',elif(Level,OldEndif,Mode)),
	 nb_setval('$endif',endif(Level,OldEndif,Mode))
	;
	 % we are now in skip, but can start an elif.
	 nb_setval('$if_skip_mode',skip)
	).

'$else'(top) :- !, fail.
'$else'(_) :-
	nb_getval('$if_level',0), !,
	'$do_error'(context_error(no_if),(:- else)).
% we have done an if, so just skip
'$else'(_) :-
	nb_getval('$endif',endif(_,_,_)), !,
	nb_setval('$if_skip_mode',skip).
% we can try the elif
'$else'(_) :-
	nb_getval('$if_level',Level),
	nb_getval('$endif',elif(Level,OldEndif,Mode)),
	nb_setval('$endif',endif(Level,OldEndif,Mode)),
	nb_setval('$if_skip_mode',run).

'$elif'(_,top) :- !, fail.
'$elif'(Goal,_) :-
	nb_getval('$if_level',0),
	'$do_error'(context_error(no_if),(:- elif(Goal))).
% we have done an if, so just skip
'$elif'(_,_) :-
	 nb_getval('$endif',endif(_,_,_)), !,
	 nb_setval('$if_skip_mode',skip).
% we can try the elif
'$elif'(Goal,_) :-
	nb_getval('$if_level',Level),
	nb_getval('$endif',elif(Level,OldEndif,Mode)),
	('$if_call'(Goal)
	    ->
% we will not skip, and we will not run any more branches.
	 nb_setval('$endif',endif(Level,OldEndif,Mode)),
	 nb_setval('$if_skip_mode',run)
	;
% we will (keep) on skipping
	 nb_setval('$if_skip_mode',skip)
	).
'$elif'(_,_).

'$endif'(top) :- !, fail.
'$endif'(_) :-
% unmmatched endif.
	nb_getval('$if_level',0),
	'$do_error'(context_error(no_if),(:- endif)).
'$endif'(_) :-
% back to where you belong.
	nb_getval('$if_level',Level),
	nb_getval('$endif',Endif),
	Level0 is Level-1,
	nb_setval('$if_level',Level0),
	arg(2,Endif,OldEndif),
	arg(3,Endif,OldMode),
	nb_setval('$endif',OldEndif),
	nb_setval('$if_skip_mode',OldMode).


'$if_call'(G) :-
	catch('$eval_if'(G), E, (print_message(error, E), fail)).

'$eval_if'(Goal) :-
	expand_term(Goal,TrueGoal),
	once(TrueGoal).

'$if_directive'((:- if(_))).
'$if_directive'((:- else)).
'$if_directive'((:- elif(_))).
'$if_directive'((:- endif)).

'$comp_mode'(OldCompMode, CompMode) :-
	(
	 nb_getval('$assert_all',on)
	->
	 OldCompMode = assert_all
	;
	 '$access_yap_flags'(11,1)
	->
	 OldCompMode = source
	;
	 OldCompMode = compile
	),
	(
	 var(CompMode) ->
	 true
	;
	 CompMode == assert_all
	->
	 nb_setval('$assert_all',on)
	;
	 CompMode == source
	->
	 nb_setval('$assert_all',off),
	 '$set_yap_flags'(11,1)
	;
	 nb_setval('$assert_all',off),
	 '$set_yap_flags'(11,0)
	).