Prolog: ignore unwanted variables in the output

Question

Is there a way to force the prolog CLI to return query results with only the variables I'm interested in? A simple example would be:

?- mother(M, C1), father(F, C1).

which returns bindings for all M, F and C1. But I'm interested only in M and F, while C1 is just clutter. In this simple example it's not bad but for longer queries with many helper variables it's much more vexing.

Is there a simple way to express that via the query; I mean without defining a separate rule?

Cheers, Jacek

Answer 1

A very straight-forward way to do this is to use library(lambda) by Ulrich Neumerkel.

For example, in your case, given the sample facts:

mother_child(m, c).
father_child(f, c).

We get for your query:

?- mother_child(M, C),
   father_child(F, C).
M = m,
C = c,
F = f.

We would like to project away the variable C.

So we wrap the whole query inside a lambda expression such that only M and F have global scope and hence are reported by the toplevel:

?- M^F+\(mother_child(M, C),
         father_child(F, C)).
M = m,
F = f.

This obviously becomes all the more useful the more variables you want to project away. You only need to specify the variables you want the toplevel to report.

Answer 2

In the case of SWI-Prolog, it offers a flag to hide variables that start with an underscore. To change the settings execute goal

set_prolog_flag(toplevel_print_anon, false).

in SWI-Prolog session. Alternatively, you can add it to your .swiplrc.

More detailed answer with examples is provided at https://stackoverflow.com/a/34917391/2471388.