[PPL-devel] Any Prolog language lawyer out there?

[Roberto Bagnara]

I have just been bitten by a problem that seems to be caused
by the different ways different Prolog systems parse the
string "+4*a".  Here are the results of a little experiment:

$ ciao
Ciao-Prolog 1.10 #5: Sat Aug 7 14:07:02 CEST 2004
?- write_canonical(+4*a).
*(+(4),a)
yes
?-


$ gprolog
GNU Prolog 1.2.18
By Daniel Diaz
Copyright (C) 1999-2004 Daniel Diaz
| ?- write_canonical(+4*a).
*(+(4),a)

yes
| ?-


$ sicstus
SICStus 3.11.1 (x86-linux-glibc2.3): Fri Feb 20 18:38:25 CET 2004
Licensed to math.unipr.it
| ?- write_canonical(+4*a).
*(4,a)
yes
| ?-


$ pl
Welcome to SWI-Prolog (Multi-threaded, Version 5.4.1)
Copyright (c) 1990-2003 University of 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.

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

?- write_canonical(+4*a).
*(4, a)

Yes
?-


$ xsb
[xsb_configuration loaded]
[sysinitrc loaded]
[packaging loaded]

XSB Version 2.6 (Duff) of June 24, 2003
[i686-pc-linux-gnu; mode: optimal; engine: slg-wam; gc: indirection; scheduling: local]

| ?- write_canonical(+4*a).
+(*(4,a))
yes
| ?-


$ yap
[ Restoring file /usr/local/lib/Yap/startup ]
[ YAP version Yap-4.5.1 ]
    ?- write_canonical(+4*a).
*(4,a)yes
    ?-

Am I correct if I say that all the 6 tested systems are /not/ violating
the ISO Prolog standard because unary plus is not mentioned in its Table 7
(p. 19) and thus its presence and priority are implementation-defined?
All the best,

     Roberto

P.S.  Standard or not standard, I find all this quite striking.

-- 
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara at cs.unipr.it