datalog: dl-walk handles circular substitutions without infinite loop (257/257)

Bug: dl-walk would infinite-loop on a circular substitution
(e.g. A→B and B→A simultaneously). The walk endlessly chased
the cycle. This couldn't be produced through dl-unify (which has
cycle-safe behavior via existing bindings), but raw dl-bind calls
or external manipulation of the subst dict could create it.

Fix: dl-walk now threads a visited-names list through the
recursion. If a variable name is already in the list, the walk
stops and returns the current term unchanged. Normal chained
walks are unaffected (A→B→C→42 still resolves to 42).

1 new unify test verifies circular substitutions don't hang.

lib/datalog/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "lang": "datalog",
-  "total_passed": 256,
+  "total_passed": 257,
   "total_failed": 0,
-  "total": 256,
+  "total": 257,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
-    {"name":"unify","passed":28,"failed":0,"total":28},
+    {"name":"unify","passed":29,"failed":0,"total":29},
     {"name":"eval","passed":38,"failed":0,"total":38},
     {"name":"builtins","passed":23,"failed":0,"total":23},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T21:16:53+00:00"
+  "generated": "2026-05-11T07:20:07+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,12 +1,12 @@
 # datalog scoreboard
 
-**256 / 256 passing** (0 failure(s)).
+**257 / 257 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 30 | 30 | ok |
 | parse | 22 | 22 | ok |
-| unify | 28 | 28 | ok |
+| unify | 29 | 29 | ok |
 | eval | 38 | 38 | ok |
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |

lib/datalog/tests/unify.sx

@@ -129,6 +129,15 @@
             ((s2 (dl-unify (quote Y) (quote tom) s1)))
             (dl-walk (quote X) s2)))
         (quote tom))
+
+      ;; Walk with circular substitution must not infinite-loop.
+      ;; Cycles return the current term unchanged.
+      (dl-ut-test!
+        "walk circular subst no hang"
+        (let ((s (dl-bind (quote B) (quote A)
+                   (dl-bind (quote A) (quote B) (dl-empty-subst)))))
+          (dl-walk (quote A) s))
+        (quote A))
       (dl-ut-test!
         "apply subst on tuple"
         (let

lib/datalog/unify.sx

@@ -33,16 +33,28 @@
 ;; variable. The result is either a non-variable term or an unbound var.
 (define
   dl-walk
+  (fn (term subst) (dl-walk-aux term subst (list))))
+
+;; Internal: walk with a visited-var set so circular substitutions
+;; (from raw dl-bind misuse) don't infinite-loop. Cycles return the
+;; current term unchanged.
+(define
+  dl-walk-aux
   (fn
-    (term subst)
+    (term subst visited)
     (if
       (dl-var? term)
       (let
         ((name (symbol->string term)))
-        (if
-          (and (dict? subst) (has-key? subst name))
-          (dl-walk (get subst name) subst)
-          term))
+        (cond
+          ((dl-member? name visited) term)
+          ((and (dict? subst) (has-key? subst name))
+            (let ((seen (list)))
+              (do
+                (for-each (fn (v) (append! seen v)) visited)
+                (append! seen name)
+                (dl-walk-aux (get subst name) subst seen))))
+          (else term)))
       term)))
 
 ;; Bind a variable symbol to a value in subst, returning a new subst.