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/* $Id$
Part of SWI-Prolog
Author: Jan Wielemaker
E-mail: J.Wielemaker@uva.nl
WWW: http://www.swi-prolog.org
Copyright (C): 2008, 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(pure_input,
[ phrase_from_file/2, % :Grammar, +File
phrase_from_file/3, % :Grammar, +File, +Options
stream_to_lazy_list/2
]).
:- use_module(library(option)).
/** <module> Pure Input from files
This module is part of pio.pl, dealing with _pure_ _input_: processing
input streams from the outside world using pure predicates, notably
grammar rules (DCG). Using pure predicates makes non-deterministic
processing of input much simpler.
Pure input uses coroutining (freeze/2) to read input from the external
source into a list _|on demand|_. The overhead of lazy reading is more
than compensated for by using block reads based on read_pending_input/3.
@tbd Provide support for alternative input readers, e.g. reading
terms, tokens, etc.
@tbd Support non-repositioning streams, such as sockets and pipes.
@author Ulrich Neumerkel
@author Jan Wielemaker
*/
%% phrase_from_file(:Grammar, +File) is nondet.
%
% Process the content of File using the DCG rule Grammar. The
% space usage of this mechanism depends on the length of the not
% committed part of Grammar. Committed parts of the temporary list
% are reclaimed by the garbage collector, while the list is
% extended on demand. Here is a very simple definition for
% searching a string in a file:
%
% ==
% ... --> []|[_],... .
%
% file_contains(File, Pattern) :-
% phrase_from_file((..., Pattern, ...), File).
%
% match_count(File, Pattern, Count) :-
% findall(x, file_contains(File, Pattern), Xs),
% length(Xs, Count).
% ==
%
% This can be called as (note that the pattern must be a string
% (code list)):
%
% ==
% ?- match_count('pure_input.pl', "file", Count).
% ==
:- meta_predicate
phrase_from_file(:, +),
phrase_from_file(:, +, +).
phrase_from_file(Grammar, File) :-
phrase_from_file(Grammar, File, []).
%% phrase_from_file(:Grammar, +File, +Options) is nondet.
%
% As phrase_from_file/2, providing additional Options. Options are
% passed to open/4, except for =buffer_size=, which is passed to
% set_stream/2. If not specified, the default buffer size is 512
% bytes. Of particular importance are the open/4 options =type=
% and =encoding=.
phrase_from_file(Grammar, File, Options) :-
strip_module(Grammar, M, G),
( select_option(buffer_size(BS), Options, OpenOptions)
-> true
; BS=512,
OpenOptions = Options
),
qphrase_file(M:G, File, BS, OpenOptions).
qphrase_file(QGrammar, File, BS, Options) :-
setup_call_cleanup(open(File, read, In, Options),
qphrase_stream(QGrammar, In, BS),
close(In)).
qphrase_stream(QGrammar, In, BuffserSize) :-
set_stream(In, buffer_size(BuffserSize)),
stream_to_lazy_list(In, List),
phrase(QGrammar, List).
%% stream_to_lazy_list(+Stream, -List) is det.
%
% Create a lazy list representing the character codes in Stream.
% It must be possible to reposition Stream. List is a list that
% ends in a delayed goal. List can be unified completely
% transparent to a (partial) list and processed transparently
% using DCGs, but please be aware that a lazy list is not the same
% as a materialized list in all respects.
%
% Typically, this predicate is used as a building block for more
% high level safe predicates such as phrase_from_file/2.
%
% @tbd Enhance of lazy list throughout the system.
stream_to_lazy_list(Stream, List) :-
stream_property(Stream, position(Pos)),
freeze(List, read_to_input_stream(Stream, Pos, List)).
read_to_input_stream(Stream, Pos, List) :-
set_stream_position(Stream, Pos),
( at_end_of_stream(Stream)
-> List = []
; read_pending_input(Stream, List, Tail),
stream_to_lazy_list(Stream, Tail)
).
/*******************************
* IDE *
*******************************/
% Using meta_predicate phrase_from_file(2, +) is inappropriate because
% the first argument is an arbitrary grammar. With :, we make less
% promises. We tell the IDE that we call G+2. This is not complete and
% should be fixed at some point.
:- multifile
prolog:called_by/2.
prolog:called_by(phrase_from_file(G, _File), [G+2]).
prolog:called_by(phrase_from_file(G, _File, _Options), [G+2]).
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