Merge pull request #1045 from triska/phrase_to_file

ADDED: Rudimentary version of phrase_to_file/2 in library(pio)

src/lib/pio.pl

@@ -1,14 +1,17 @@
 :- module(pio, [phrase_from_file/2,
-                phrase_from_file/3]).
+                phrase_from_file/3,
+                phrase_to_file/2]).
 
 :- use_module(library(dcgs)).
 :- use_module(library(error)).
 :- use_module(library(freeze)).
 :- use_module(library(iso_ext), [setup_call_cleanup/3, partial_string/3]).
 :- use_module(library(lists), [member/2]).
+:- use_module(library(format), [format/3]).
 
-:- meta_predicate phrase_from_file(2, ?).
+:- meta_predicate(phrase_from_file(2, ?)).
-:- meta_predicate phrase_from_file(2, ?, ?).
+:- meta_predicate(phrase_from_file(2, ?, ?)).
+:- meta_predicate(phrase_to_file(2, ?)).
 
 phrase_from_file(NT, File) :-
     phrase_from_file(NT, File, []).
@@ -43,3 +46,29 @@ reader_step(Stream, Pos, Xs0) :-
             partial_string(Cs, Xs0, Xs),
             stream_to_lazy_list(Stream, Xs)
         ).
+
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   phrase_to_file(+GRBody, +File)
+
+   Emit the list of characters described by the grammar rule body
+   GRBody to File. File is a string, which is also the representation
+   used by library(files) to test for existence etc. of files.
+
+   An ideal implementation of phrase_to_file/2 writes each character
+   as soon as it becomes known and no choice-points remain, and thus
+   avoids the manifestation of the entire string in memory. See #691
+   for more information.
+
+   The current preliminary implementation is provided so that Prolog
+   programmers can already get used to describing output with DCGs,
+   and then writing it to a file when necessary. This simple
+   implementation suffices as long as the entire contents can be
+   represented in memory, and thus covers a large number of use cases.
+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+phrase_to_file(GRBody, File) :-
+        atom_chars(Atom, File),
+        phrase(GRBody, Chars),
+        setup_call_cleanup(open(Atom, write, Stream),
+                           format(Stream, "~s", [Chars]),
+                           close(Stream)).