Getting connected components of graph in Prolog

Question

I'm struggling with logic programming. I have a this problem and I hope some of you can help me with it. Discontinous graph is represented by facts in this way:

h(0,1).
h(1,2).
h(3,4).
h(3,5).

So there is two separate graph components. I would like all the separate components on the output represented by a list. So if there is three separate components in the graph, there will be three lists. For the given example above, the expected output is [[0,1,2],[3,4,5]].

Answer 1

Using iwhen/2 we can define binrel_connected/2 like so:

:- use_module(library(ugraphs)).
:- use_module(library(lists)).

binrel_connected(R_2, CCs) :-
   findall(X-Y, call(R_2,X,Y), Es),
   iwhen(ground(Es), ( vertices_edges_to_ugraph([],Es,G0),
                       reduce(G0,G),
                       keys_and_values(G,CCs,_) )).

Sample query on SICStus Prolog 4.5.0 with symm/2 for symmetric closure:

| ?- binrel_connected(symm(h), CCs).
CCs = [[0,1,2],[3,4,5]] ? ;
no