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.