:- module(dialect,
	  [
	   exists_source/1,
	   source_exports/2
	  ]).


prolog:'$expects_dialect'(yap) :- !,
	eraseall('$dialect'),
	recorda('$dialect',yap,_).
prolog:'$expects_dialect'(Dialect) :-
	check_dialect(Dialect), 
	eraseall('$dialect'),
	load_files(library(dialect/Dialect),[silent(true),if(not_loaded)]),
	(   current_predicate(Dialect:setup_dialect/0)
	->  Dialect:setup_dialect
	;   true
	),
	recorda('$dialect',Dialect,_).		

check_dialect(Dialect) :-
	var(Dialect),!,
	'$do_error'(instantiation_error,(:- dialect(Dialect))).
check_dialect(Dialect) :-
	\+ atom(Dialect),!,
	'$do_error'(type_error(Dialect),(:- dialect(Dialect))).
check_dialect(Dialect) :-
	exists_source(library(dialect/Dialect)), !.
check_dialect(Dialect) :-
	'$do_error'(domain_error(dialect,Dialect),(:- dialect(Dialect))).

%%	exists_source(+Source) is semidet.
%
%	True if Source (a term  valid   for  load_files/2) exists. Fails
%	without error if this is not the case. The predicate is intended
%	to be used with  :-  if,  as   in  the  example  below. See also
%	source_exports/2.
%
%	==
%	:- if(exists_source(library(error))).
%	:- use_module_library(error).
%	:- endif.
%	==

%exists_source(Source) :-
%	exists_source(Source, _Path).

exists_source(Source, Path) :-
	absolute_file_name(Source, Path,
			   [ file_type(prolog),
			     access(read),
			     file_errors(fail)
			   ]).

%%	source_exports(+Source, +Export) is semidet.
%%	source_exports(+Source, -Export) is nondet.
%
%	True if Source exports Export. Fails   without  error if this is
%	not the case.  See also exists_source/1.
%
%	@tbd	Should we also allow for source_exports(-Source, +Export)?

source_exports(Source, Export) :-
	open_source(Source, In),
	catch(call_cleanup(exports(In, Exports), close(In)), _, fail),
	(   ground(Export)
	->  memberchk(Export, Exports)
	;   member(Export, Exports)
	).

%%	open_source(+Source, -In:stream) is semidet.
%
%	Open a source location.

open_source(File, In) :-
	exists_source(File, Path),
	open(Path, read, In),
	(   peek_char(In, #)
	->  skip(In, 10)
	;   true
	).

exports(In, Exports) :-
	read(In, Term),
	Term = (:- module(_Name, Exports)).