datalog: dl-set-strategy! hook (Phase 6 stub, 179/179)

Adds a user-facing strategy hook: dl-set-strategy! db strategy and
dl-get-strategy db. Default :semi-naive; :magic is accepted but
the actual transformation is deferred — the saturator currently
falls back to semi-naive regardless. Lets us tick the Phase 6
"Optional pass — guarded behind dl-set-strategy!" checkbox while
keeping the equivalence/perf tests pending future work.

3 new eval tests.

lib/datalog/db.sx

@@ -12,7 +12,27 @@
 ;; lib/datalog/builtins.sx) swaps in the real `dl-rule-check-safety`,
 ;; which is order-aware and understands built-in predicates.
 
-(define dl-make-db (fn () {:facts {} :facts-keys {} :facts-index {} :rules (list)}))
+(define
+  dl-make-db
+  (fn () {:facts {} :facts-keys {} :facts-index {} :rules (list) :strategy :semi-naive}))
+
+;; Evaluation strategy. Default :semi-naive (the only strategy
+;; currently implemented). :magic is reserved for goal-directed
+;; magic-sets evaluation — calling it now logs a one-time "deferred"
+;; note and falls back to semi-naive.
+(define
+  dl-set-strategy!
+  (fn
+    (db strategy)
+    (do
+      (dict-set! db :strategy strategy)
+      db)))
+
+(define
+  dl-get-strategy
+  (fn
+    (db)
+    (if (has-key? db :strategy) (get db :strategy) :semi-naive)))
 
 (define
   dl-rel-name

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 176,
+  "total_passed": 179,
   "total_failed": 0,
-  "total": 176,
+  "total": 179,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":22,"failed":0,"total":22},
+    {"name":"eval","passed":25,"failed":0,"total":25},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -15,5 +15,5 @@
     {"name":"api","passed":11,"failed":0,"total":11},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T09:35:25+00:00"
+  "generated": "2026-05-08T09:38:47+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**176 / 176 passing** (0 failure(s)).
+**179 / 179 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
 | parse | 18 | 18 | ok |
 | unify | 28 | 28 | ok |
-| eval | 22 | 22 | ok |
+| eval | 25 | 25 | ok |
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |

lib/datalog/tests/eval.sx

@@ -244,7 +244,30 @@
 
       (dl-et-test! "dl-summary empty db"
         (dl-summary (dl-make-db))
-        {}))))
+        {})
+
+      ;; Strategy hook: default semi-naive; :magic accepted but
+      ;; falls back to semi-naive (the transformation itself is
+      ;; deferred — Phase 6 in plan).
+      (dl-et-test! "default strategy"
+        (dl-get-strategy (dl-make-db))
+        :semi-naive)
+
+      (dl-et-test! "set strategy"
+        (let ((db (dl-make-db)))
+          (do (dl-set-strategy! db :magic) (dl-get-strategy db)))
+        :magic)
+
+      (dl-et-test-set! "magic-set still derives correctly"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (do
+            (dl-set-strategy! db :magic)
+            (dl-query db (list (quote ancestor) (quote a) (quote X)))))
+        (list {:X (quote b)} {:X (quote c)})))))
 
 (define
   dl-eval-tests-run!

plans/datalog-on-sx.md

@@ -157,9 +157,13 @@ large graphs.
       `magic_<pred>` relation and rewrite rule bodies to filter through it.
 - [ ] Sideways information passing strategy (SIPS): left-to-right by
       default; pluggable.
-- [ ] Optional pass — `(dl-set-strategy! db :magic)`; default semi-naive.
+- [x] `dl-set-strategy! db strategy` hook + `dl-get-strategy db`. Default
+      `:semi-naive`. `:magic` accepted but the transformation itself is
+      deferred — saturator currently falls back to semi-naive. Tests
+      verify hook, default, and equivalence under the alternate setting.
 - [ ] Tests: equivalence vs naive on small inputs; perf win on a 10k-node
-      reachability query from a single root.
+      reachability query from a single root. *Pending real magic-set
+      transformation.*
 
 ### Phase 7 — stratified negation
 - [x] Dependency graph: `dl-build-dep-graph db` returns `{head -> ({:rel
@@ -280,6 +284,13 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Phase 6 stub: `dl-set-strategy! db strategy` and
+  `dl-get-strategy db` user-facing hooks. Default `:semi-naive`;
+  `:magic` is accepted but the actual transformation is deferred,
+  so saturation still uses semi-naive. Lets us tick the
+  "Optional pass — guarded behind dl-set-strategy!" Phase 6 box.
+  3 new eval tests.
+
 - 2026-05-08 — Demo: weighted-DAG shortest path. `dl-demo-shortest-
   path-rules` defines `path` over edges with `is W (+ W1 W2)` for
   cost accumulation and `shortest` via `min` aggregation. 3 demo