Find set of solutions and terminate in SWI-Prolog?

user_1256041 0
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Here is my program:

reachable(X) :- start(X).
reachable(X) :- link(Y, X), reachable(Y).

start(london).

link(london, paris).
link(paris, london).
link(london, berlin).
link(paris, berlin).
link(berlin, paris).

Now if I query it, I get solutions:

?- reachable(X).
X = london ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
X = paris ;
...

Why does SWI-Prolog keep finding the same solutions?

How can I fix my program to return the set of solutions then halt?

Here's what I am looking for:

?- reachable(X).
X = paris ;
X = london ;
X = berlin ;
No

$ swipl --version  
SWI-Prolog version 7.6.4 for amd64
prolog
1 Answers

The simplest solution is to use tabled execution:

% The next line is required for version 7.6.4, but is considered 
% deprecated in the latest version of the SWI-Prolog compiler!

:- use_module(library(tabling)).

% The next line prepares the predicate reachable/1 for tabled execution!

:- table reachable/1.

reachable(X) :- start(X).
reachable(X) :- link(Y, X), reachable(Y).

start(london).

link(london, paris).
link(paris, london).
link(london, berlin).
link(paris, berlin).
link(berlin, paris).

Example:

?- reachable(X).
X = berlin ;
X = paris ;
X = london.
user_115359400
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