I've been puzzled by a modelisation problem for some time and I have to confess I have no idea how I could "properly" solve it in core.logic.
It is very easy to state: given a tree (acyclic unidirectional oriented graph) and a vertex in it, how do you use core.logic to define a goal which allow a lvar to be any reachable vertex from the given vertex?
I've started out with something as simple as possible:
(defrel vertex x)
(defrel child)
(def facts
(pldb/db
[vertex 'a]
[vertex 'b] [child 'a 'b]
[vertex 'c] [child 'b 'c]
[vertex 'd] [child 'c 'd]))
Given this configuration, I aim at defining a goal which allows a lvar to take values in ['a 'b 'c 'd]. It's straightforward to get reachable vertices with "1 hop":
(defn reachableo
[a]
(fresh [q]
(child a q)))
and you can add variables for 2 hops and so on but... can it be generalized? I've thought to define a list of lvar with something like
(let [vars (repeatedly lvar)]
(map #(child (nth vars %) (nth vars (inc %)))
(-> vars count dec range))
(all
...))
but several attempts later, I must confess I'm not sure it's the correct way to go.
(pldb/db-rel vertex x)
(pldb/db-rel child parent child)
(def facts
(pldb/db
[vertex 'a]
[vertex 'b] [child 'a 'b]
[vertex 'c] [child 'b 'c]
[vertex 'd] [child 'c 'd]))
The recursive goal:
(defn reachable° [from to]
(l/fresh [v]
(l/conde
[(child from to)]
[(child from v) (reachable° v to)])))
Testing
=> (pldb/with-db facts
(vec (l/run* [to] (reachable° 'a to))))
[b c d]