I wrote this goal:
(use 'clojure.core.logic)
(use 'clojure.core.logic.protocols)
(defn pluso [t1 t2 s]
(fn goal [a]
(let [args (map (partial walk a) [t1 t2 s])
fresh? (map lvar? args)
ground? (map not fresh?)
[t1 t2 s] args]
(cond
(= [true true true ] ground?) (if (= s (+ t1 t2)) a nil)
(= [true true false] ground?) (unify a s (+ t1 t2))
(= [true false true ] ground?) (unify a t2 (- s t1))
(= [false true true ] ground?) (unify a t1 (- s t2))
:else
a ;; Returning the substitution map will mark this goal as successful
;; while it is neither successful nor failed. The fresh variables
;; might be ground later.
))))
When I run this query I get (_0) while I'm expecting (3). The root of the
problem is the returning of the substitution map if more than one variable
is not ground.
(run* [q]
(fresh [a]
(pluso q 4 a)
(== a 7)))
I've tried returning a choice but this leads to an endless loop.
Is it at all possible to somehow adapt the goal to await the grounding of
the a-variable? What if the fresh variables are never ground?
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