datalog: dl-eval-magic single-call magic-sets entry (206/206)

Symmetric to dl-eval but routes single-positive-literal queries
through dl-magic-query for goal-directed evaluation. Multi-literal
query bodies fall back to standard dl-query (magic-sets is wired
for single goals only).

  (dl-eval-magic source-string "?- ancestor(a, X).")

1 new api test.

lib/datalog/api.sx

@@ -169,6 +169,35 @@
         (else
           (dl-query db (get (first queries) :query)))))))
 
+;; (dl-eval-magic source query-source) — like dl-eval but routes a
+;; single-positive-literal query through `dl-magic-query` for goal-
+;; directed evaluation. Multi-literal query bodies fall back to the
+;; standard dl-query path (magic-sets is currently only wired for
+;; single-positive goals). The caller's source is parsed afresh
+;; each call so successive invocations are independent.
+(define
+  dl-eval-magic
+  (fn
+    (source query-source)
+    (let
+      ((db (dl-program source))
+        (queries (dl-parse query-source)))
+      (cond
+        ((= (len queries) 0) (error "dl-eval-magic: query string is empty"))
+        ((not (has-key? (first queries) :query))
+          (error
+            "dl-eval-magic: second arg must be a `?- ...` query clause"))
+        (else
+          (let
+            ((qbody (get (first queries) :query)))
+            (cond
+              ((and (= (len qbody) 1)
+                    (list? (first qbody))
+                    (> (len (first qbody)) 0)
+                    (symbol? (first (first qbody))))
+                (dl-magic-query db (first qbody)))
+              (else (dl-query db qbody)))))))))
+
 (define
   dl-rule-head-rels
   (fn