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I have the following database:

vegetarian(jose).
vegetarian(james).
vegetable(carrot).
vegetable(egg_plant).
likes(jose,X):-vegetable(X).
loves(Who,egg_plant):-vegetarian(Who).

When I do the query vegetarian(_). I was expecting to get _ = jose; _ = james. but I am instead getting true; true.

If I instead do vegetarian(X)., then I get the expected answer X = jose; X = james. Why this difference?

rhododendron
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  • Which Prolog are you using? – Peter Ludemann Jan 02 '21 at 07:15
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    Pretty sure this is a feature of the Prolog command line (i.e. "the Prolog toplevel"). As you explicitly have stated that you are not interested in the value of certain variables (by using `_` as name), the toplevel won't bother to print the term denoted by `_`. – David Tonhofer Jan 02 '21 at 09:27

1 Answers1

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If you're using SWI-Prolog, you can control this with the flag toplevel_print_anon). However, a name consisting of a single underscore (_) is special and never prints.

$ swipl dd.pl
Welcome to SWI-Prolog (threaded, 64 bits, version 8.3.16)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

For online help and background, visit https://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).

?- set_prolog_flag(toplevel_print_anon, true).
true.

?- vegetarian(_X).
_X = jose ;
_X = james.

?- vegetarian(_).
true ;
true.

?- set_prolog_flag(toplevel_print_anon, false).
true.

?- vegetarian(_X).
true ;
true.
Peter Ludemann
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