/*  $Id$

    Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        wielemak@science.uva.nl
    WWW:           http://www.swi-prolog.org
    Copyright (C): 2007, University of Amsterdam

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

    As a special exception, if you link this library with other files,
    compiled with a Free Software compiler, to produce an executable, this
    library does not by itself cause the resulting executable to be covered
    by the GNU General Public License. This exception does not however
    invalidate any other reasons why the executable file might be covered by
    the GNU General Public License.
*/

:- module(prolog_dialect,
	  [ expects_dialect/1,		% +Dialect
	    exists_source/1,		% +Source
	    source_exports/2		% +Source, ?Export
	  ]).
:- use_module(library(error)).
:- use_module(library(lists)).

/**	<module> Support multiple Prolog dialects

The idea for this predicate  was  raised   by  Vitor  Santos  Costa in a
discussion to reach as a portability   framework  between SWI-Prolog and
YAP.

This library defines :- expects_dialect/1, telling  the system for which
Prolog dialect was written,  as  well   as  useful  tests in conditional
compilation:

	* exists_source/1
	* source_exports/2

@see	if/1, require/1, term_expansion/2, goal_expansion/2.
@author	Jan Wielemaker
@author Vitor Santos Costa
*/

%%	expects_dialect(+Dialect:atom) is det.
%
%	Tell Prolog all subsequent code to the   end  of the file or the
%	next :- expects_dialect/1 directive is written for the indicated
%	Dialect.   The   current   dialect     is    available   through
%	prolog_load_context/2.
%	
%	@tbd	Should we setup the dialect module only as autoload for
%		the current module?

expects_dialect(Dialect) :-
	must_be(atom, Dialect),
	'$set_dialect'(_, Dialect),
	(   Dialect == swi
	->  true
	;   attach_dialect(Dialect)
	).


attach_dialect(Dialect) :-
	exists_source(library(dialect/Dialect)), !,
	prolog_load_context(module, Module),
	use_module(Module:library(dialect/Dialect)).
attach_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)).

