datalog: aggregate validates that agg-var appears in goal

`count(N, Y, p(X))` silently returned `N = 1` because `Y` was never
bound by the goal — every match contributed the same unbound symbol
which dl-val-member? deduped to a single entry. Similarly:

  sum(S, Y, p(X))    => raises "expected number, got symbol"
  findall(L, Y, p(X)) => L = (Y)  (a list containing the unbound symbol)
  count(N, Y, p(X))   => N = 1    (silent garbage)

Added a third validator in dl-eval-aggregate: the agg-var must
syntactically appear among the goal's variables. Error names the
variable and the goal and explains why the result would be
meaningless.

1 new test; conformance 263/263.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/aggregates.sx

@@ -113,6 +113,14 @@
           (error (str "datalog aggregate (" op
                       "): third arg must be a positive literal, got "
                       goal)))
+        ((not (dl-member-string?
+                (symbol->string agg-var)
+                (dl-vars-of goal)))
+          (error (str "datalog aggregate (" op
+                      "): aggregation variable " agg-var
+                      " does not appear in the goal " goal
+                      " — without it every match contributes the same "
+                      "(unbound) value and the result is meaningless")))
         (else
           (let ((vals (list)))
             (do

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 262,
+  "total_passed": 263,
   "total_failed": 0,
-  "total": 262,
+  "total": 263,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
@@ -11,10 +11,10 @@
     {"name":"builtins","passed":23,"failed":0,"total":23},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
-    {"name":"aggregates","passed":22,"failed":0,"total":22},
+    {"name":"aggregates","passed":23,"failed":0,"total":23},
     {"name":"api","passed":22,"failed":0,"total":22},
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:50:41+00:00"
+  "generated": "2026-05-11T07:56:45+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**262 / 262 passing** (0 failure(s)).
+**263 / 263 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -11,7 +11,7 @@
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
-| aggregates | 22 | 22 | ok |
+| aggregates | 23 | 23 | ok |
 | api | 22 | 22 | ok |
 | magic | 36 | 36 | ok |
 | demo | 21 | 21 | ok |

lib/datalog/tests/aggregates.sx

@@ -256,6 +256,19 @@
           (fn () (dl-eval "p(1). q(N) :- count(N, X, 42)." "?- q(N).")))
         true)
 
+      ;; Aggregate validates that the agg-var (2nd arg) appears in the
+      ;; goal. Without it every match contributes the same unbound
+      ;; symbol — count silently returns 1, sum raises a confusing
+      ;; "expected number" error, etc. Catch the mistake at safety
+      ;; check time instead.
+      (dl-at-test! "agg-var must appear in goal"
+        (dl-at-throws?
+          (fn ()
+            (dl-eval
+              "p(1). p(2). c(N) :- count(N, Y, p(X))."
+              "?- c(N).")))
+        true)
+
       ;; Indirect recursion through aggregation also rejected.
       ;; q -> r (via positive lit), r -> q (via aggregate body).
       ;; The aggregate edge counts as negation for stratification.