Adding new facts to a file in Prolog

Question

I have a problem in Prolog regarding adding new facts to the file 'relations.pl'. Every time I get facts I save them and I use

tell('relations.pl').
listing(relation).
told.

The only problem is that I want to insert the new facts and avoid storing the same facts more that one if there are any.

Is there anyway to do this? Thank you,

You could append text to a file, but be warned, this easily leads to a lot of errors and inconsistencies. Instead write it all at once. — false, Jun 29 '14 at 16:15

score 1 · Accepted Answer · answered Jun 29 '14 at 16:32

It is a bit more robust to say out_tofile(listing(relation),'relation.pl'). The only inplace operation for textfiles is to append new text to them. I cannot recommend doing this here. Appending would be fine for logfiles.

:- meta_predicate
      out_tofile(0,+),             % out_tofile(:,+) in older versions
      out_ontofile(0,+),           % idem
      out_tostream__andclose(0,+). % idem

out_tofile(Goal, File) :-
   open(File,write,Stream),
   out_tostream__andclose(Goal, Stream).

out_ontofile(Goal, File) :-
   open(File,append,Stream),
   out_tostream__andclose(Goal, Stream).

out_tostream__andclose(Goal, Stream) :-
   current_output(Stream0),
   call_cleanup((set_output(Stream),once(Goal)), set_output_close(Stream0, Stream)).

set_output_close(Stream0, Stream) :-
   set_output(Stream0),
   close(Stream).

Adding new facts to a file in Prolog

1 Answers1