Do SWI-Prolog files need start and end characters?

2
votes

I'm new to SWI-Prolog and am trying some tutorials. Every file I try to load through the command line, however, gets 2 error messages - one at the start (Operator expected) and one at the end (Unexpected end of file). Files are saved in the same directory as the one I'm working in.

For example, I have this file saved as kb2.pl

listensToMusic(mia).  
happy(yolanda).   
playsAirGuitar(mia) :- listensToMusic(mia).   
playsAirGuitar(yolanda) :- listensToMusic(yolanda).
listensToMusic(yolanda):- happy(yolanda).

From the start of opening Prolog (through /opt/local/bin/swipl), my command line looks like this:

% library(swi_hooks) compiled into pce_swi_hooks 0.00 sec, 3,928 bytes
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 5.10.2)
Copyright (c) 1990-2010 University of Amsterdam, VU Amsterdam
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

help, use ?- help(Topic). or ?- apropos(Word).

?- [kb2].
ERROR: /Users/name/Desktop/kb2.pl:1:0: Syntax error: Operator expected
ERROR: /Users/name/Desktop/kb2.pl:10:52: Syntax error: Unexpected end of file
% kb2 compiled 0.00 sec, 2,584 bytes
true.

?-

When I ask for the listing of the compiled file, I get:

\happy(yolanda).
\playsAirGuitar(mia) :- listensToMusic(mia).
\playsAirGuitar(yolanda) :- listensToMusic(yolanda).
true.

So it's cutting out the first and last lines of the file.

I've searched for these error messages online and found a lot of useful hints about formatting for Prolog, but none that address this situation. Is there some sort of special character or formatting that should be used for the start and end of a file for Prolog?

2
prologsyntax-errorswi-prolog

2 Answers

1
votes

Which operating system are you on? Are you sure the file is ASCII (and not UTF-16)? Does it have the native line ending for your platform? Can you post the whole file (raw) so people can check it?

0
votes

Many Prolog systems, such as SWI-Prolog, Jekejeke Prolog, allow to detect a BOM as the first character of a text file. The BOM is some white space, that indicates the encoding of the text file:

254 255          UTF-16BE
255 254          UTF-16LE
239 187 191      UTF-8

So there is no need to restrict yourself to ASCII. Many text editors will place the BOM for you. And SWI-Prolog, Jekejeke Prolog, by default probe the BOM.