graph_test.chr

:- module(graph_test, [run/3]).

:- chr_constraint edge/2, path/3, get_path/3, run/3.
:- chr_type vertex ---> a ; b ; c ; d.

base    @ edge(X, Y) ==> path(X, Y, 1).
step    @ path(X, Y, N), edge(Y, Z) ==> N1 is N + 1, path(X, Z, N1).
shorter @ path(X, Y, N) \ path(X, Y, M) <=> M >= N | true.

found    @ path(X, Y, N) \ get_path(X, Y, R) <=> R = found(N).
notfound @ get_path(_, _, R) <=> R = none.

run(R1, R2, R3) <=>
    edge(a, b),
    edge(b, c),
    edge(a, c),
    edge(c, d),
    get_path(a, c, R1),
    get_path(a, d, R2),
    get_path(d, a, R3).