datalog: aggregation count/sum/min/max (Phase 8, 134/134)

New lib/datalog/aggregates.sx: (count R V Goal), (sum R V Goal),
(min R V Goal), (max R V Goal). dl-eval-aggregate runs
dl-find-bindings on the goal under the outer subst, collects
distinct values of V, applies the operator, binds R. Empty input:
count/sum return 0; min/max produce no binding (rule fails).

Group-by emerges naturally from outer-subst substitution into the
goal — `popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).`
counts per-post.

Stratifier extended: dl-aggregate-dep-edge contributes a
negation-like edge so the aggregate's goal relation is fully
derived before the aggregate fires (non-monotonicity respected).
Safety relaxed for aggregates: goal-internal vars are existentials,
only the result var becomes bound.

lib/datalog/aggregates.sx

@@ -0,0 +1,139 @@
+;; lib/datalog/aggregates.sx — count / sum / min / max aggregation.
+;;
+;; Surface form (always 3-arg in the parsed AST):
+;;
+;;   (count  Result  Var  GoalLit)
+;;   (sum    Result  Var  GoalLit)
+;;   (min    Result  Var  GoalLit)
+;;   (max    Result  Var  GoalLit)
+;;
+;; Parsed naturally because arg-position compounds are already allowed
+;; (Phase 4 needs them for arithmetic). At evaluation time the aggregator
+;; runs `dl-find-bindings` on `GoalLit` under the current subst, collects
+;; the distinct values of `Var`, and binds `Result`.
+;;
+;; Aggregation is non-monotonic — `count(C, X, p(X))` shrinks as p loses
+;; tuples. The stratifier (lib/datalog/strata.sx) treats every aggregate's
+;; goal relation as a negation-like edge so the inner relation is fully
+;; derived before the aggregate fires.
+;;
+;; Empty input: count → 0, sum → 0, min/max → no binding (rule fails).
+
+(define dl-aggregate-rels (list "count" "sum" "min" "max"))
+
+(define
+  dl-aggregate?
+  (fn
+    (lit)
+    (and
+      (list? lit)
+      (>= (len lit) 4)
+      (let ((rel (dl-rel-name lit)))
+        (cond
+          ((nil? rel) false)
+          (else (dl-member-string? rel dl-aggregate-rels)))))))
+
+;; Apply aggregation operator to a list of (already-distinct) numeric or
+;; symbolic values. Returns the aggregated value, or :empty if min/max
+;; has no input.
+(define
+  dl-do-aggregate
+  (fn
+    (op vals)
+    (cond
+      ((= op "count") (len vals))
+      ((= op "sum") (dl-sum-vals vals 0))
+      ((= op "min")
+        (cond
+          ((= (len vals) 0) :empty)
+          (else (dl-min-vals vals 1 (first vals)))))
+      ((= op "max")
+        (cond
+          ((= (len vals) 0) :empty)
+          (else (dl-max-vals vals 1 (first vals)))))
+      (else (error (str "datalog: unknown aggregate " op))))))
+
+(define
+  dl-sum-vals
+  (fn
+    (vals acc)
+    (cond
+      ((= (len vals) 0) acc)
+      (else (dl-sum-vals (rest vals) (+ acc (first vals)))))))
+
+(define
+  dl-min-vals
+  (fn
+    (vals i cur)
+    (cond
+      ((>= i (len vals)) cur)
+      (else
+        (let ((v (nth vals i)))
+          (dl-min-vals vals (+ i 1) (if (< v cur) v cur)))))))
+
+(define
+  dl-max-vals
+  (fn
+    (vals i cur)
+    (cond
+      ((>= i (len vals)) cur)
+      (else
+        (let ((v (nth vals i)))
+          (dl-max-vals vals (+ i 1) (if (> v cur) v cur)))))))
+
+;; Membership check by deep equality (so 30 == 30.0 etc).
+(define
+  dl-val-member?
+  (fn
+    (v xs)
+    (cond
+      ((= (len xs) 0) false)
+      ((dl-tuple-equal? v (first xs)) true)
+      (else (dl-val-member? v (rest xs))))))
+
+;; Evaluate an aggregate body lit under `subst`. Returns the list of
+;; extended substitutions (0 or 1 element).
+(define
+  dl-eval-aggregate
+  (fn
+    (lit db subst)
+    (let
+      ((op (dl-rel-name lit))
+        (result-var (nth lit 1))
+        (agg-var (nth lit 2))
+        (goal (nth lit 3)))
+      (let
+        ((substs (dl-find-bindings (list goal) db subst))
+          (vals (list)))
+        (do
+          (for-each
+            (fn
+              (s)
+              (let ((v (dl-apply-subst agg-var s)))
+                (when
+                  (not (dl-val-member? v vals))
+                  (append! vals v))))
+            substs)
+          (let ((agg-val (dl-do-aggregate op vals)))
+            (cond
+              ((= agg-val :empty) (list))
+              (else
+                (let ((s2 (dl-unify result-var agg-val subst)))
+                  (if (nil? s2) (list) (list s2)))))))))))
+
+;; Stratification edges from aggregates: like negation, the goal's
+;; relation must be in a strictly lower stratum so that the aggregate
+;; fires only after the underlying tuples are settled.
+(define
+  dl-aggregate-dep-edge
+  (fn
+    (lit)
+    (cond
+      ((dl-aggregate? lit)
+        (let ((goal (nth lit 3)))
+          (cond
+            ((and (list? goal) (> (len goal) 0))
+              (let ((rel (dl-rel-name goal)))
+                (if (nil? rel) nil {:rel rel :neg true})))
+            (else nil))))
+      (else nil))))

lib/datalog/builtins.sx

@@ -262,6 +262,19 @@
                       missing
                       " — they must be bound by an earlier "
                       "positive body literal")))))))
+        (define
+          dl-process-agg!
+          (fn
+            (lit)
+            (let
+              ((result-var (nth lit 1)))
+              ;; Aggregate goal vars are existentially quantified within
+              ;; the aggregate; nothing required from outer context. The
+              ;; result var becomes bound after the aggregate fires.
+              (when
+                (dl-var? result-var)
+                (dl-add-bound! (list (symbol->string result-var)))))))
+
         (define
           dl-process-lit!
           (fn
@@ -271,6 +284,7 @@
               (cond
                 ((and (dict? lit) (has-key? lit :neg))
                   (dl-process-neg! lit))
+                ((dl-aggregate? lit) (dl-process-agg! lit))
                 ((dl-eq? lit) (dl-process-eq! lit))
                 ((dl-is? lit) (dl-process-is! lit))
                 ((dl-comparison? lit) (dl-process-cmp! lit))

lib/datalog/conformance.conf

@@ -9,6 +9,7 @@ PRELOADS=(
   lib/datalog/unify.sx
   lib/datalog/db.sx
   lib/datalog/builtins.sx
+  lib/datalog/aggregates.sx
   lib/datalog/strata.sx
   lib/datalog/eval.sx
 )
@@ -21,4 +22,5 @@ SUITES=(
   "builtins:lib/datalog/tests/builtins.sx:(dl-builtins-tests-run!)"
   "semi_naive:lib/datalog/tests/semi_naive.sx:(dl-semi-naive-tests-run!)"
   "negation:lib/datalog/tests/negation.sx:(dl-negation-tests-run!)"
+  "aggregates:lib/datalog/tests/aggregates.sx:(dl-aggregates-tests-run!)"
 )

lib/datalog/eval.sx

@@ -206,6 +206,7 @@
               (cond
                 ((and (dict? lit) (has-key? lit :neg))
                   (dl-match-negation (get lit :neg) db subst))
+                ((dl-aggregate? lit) (dl-eval-aggregate lit db subst))
                 ((dl-builtin? lit)
                   (let
                     ((s (dl-eval-builtin lit subst)))

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 124,
+  "total_passed": 134,
   "total_failed": 0,
-  "total": 124,
+  "total": 134,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -10,7 +10,8 @@
     {"name":"eval","passed":15,"failed":0,"total":15},
     {"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}
+    {"name":"negation","passed":10,"failed":0,"total":10},
+    {"name":"aggregates","passed":10,"failed":0,"total":10}
   ],
-  "generated": "2026-05-08T08:20:41+00:00"
+  "generated": "2026-05-08T08:28:29+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**124 / 124 passing** (0 failure(s)).
+**134 / 134 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -11,3 +11,4 @@
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
+| aggregates | 10 | 10 | ok |

lib/datalog/strata.sx

@@ -33,22 +33,30 @@
                     (for-each
                       (fn
                         (lit)
-                        (let
-                          ((target
-                             (cond
-                               ((and (dict? lit) (has-key? lit :neg))
-                                 (dl-rel-name (get lit :neg)))
-                               ((dl-builtin? lit) nil)
-                               ((and (list? lit) (> (len lit) 0))
-                                 (dl-rel-name lit))
-                               (else nil)))
-                            (neg?
-                              (and (dict? lit) (has-key? lit :neg))))
-                          (when
-                            (not (nil? target))
-                            (append!
-                              existing
-                              {:rel target :neg neg?}))))
+                        (cond
+                          ((dl-aggregate? lit)
+                            (let
+                              ((edge (dl-aggregate-dep-edge lit)))
+                              (when
+                                (not (nil? edge))
+                                (append! existing edge))))
+                          (else
+                            (let
+                              ((target
+                                 (cond
+                                   ((and (dict? lit) (has-key? lit :neg))
+                                     (dl-rel-name (get lit :neg)))
+                                   ((dl-builtin? lit) nil)
+                                   ((and (list? lit) (> (len lit) 0))
+                                     (dl-rel-name lit))
+                                   (else nil)))
+                                (neg?
+                                  (and (dict? lit) (has-key? lit :neg))))
+                              (when
+                                (not (nil? target))
+                                (append!
+                                  existing
+                                  {:rel target :neg neg?}))))))
                       (get rule :body)))))))
           (dl-rules db))
         g))))
@@ -79,6 +87,9 @@
                   (let
                     ((t
                        (cond
+                         ((dl-aggregate? lit)
+                           (let ((edge (dl-aggregate-dep-edge lit)))
+                             (if (nil? edge) nil (get edge :rel))))
                          ((and (dict? lit) (has-key? lit :neg))
                            (dl-rel-name (get lit :neg)))
                          ((dl-builtin? lit) nil)

lib/datalog/tests/aggregates.sx

@@ -0,0 +1,223 @@
+;; lib/datalog/tests/aggregates.sx — count / sum / min / max.
+
+(define dl-at-pass 0)
+(define dl-at-fail 0)
+(define dl-at-failures (list))
+
+(define
+  dl-at-deep=?
+  (fn
+    (a b)
+    (cond
+      ((and (list? a) (list? b))
+        (and (= (len a) (len b)) (dl-at-deq-l? a b 0)))
+      ((and (dict? a) (dict? b))
+        (let ((ka (keys a)) (kb (keys b)))
+          (and (= (len ka) (len kb)) (dl-at-deq-d? a b ka 0))))
+      ((and (number? a) (number? b)) (= a b))
+      (else (equal? a b)))))
+
+(define
+  dl-at-deq-l?
+  (fn
+    (a b i)
+    (cond
+      ((>= i (len a)) true)
+      ((not (dl-at-deep=? (nth a i) (nth b i))) false)
+      (else (dl-at-deq-l? a b (+ i 1))))))
+
+(define
+  dl-at-deq-d?
+  (fn
+    (a b ka i)
+    (cond
+      ((>= i (len ka)) true)
+      ((let ((k (nth ka i)))
+         (not (dl-at-deep=? (get a k) (get b k))))
+        false)
+      (else (dl-at-deq-d? a b ka (+ i 1))))))
+
+(define
+  dl-at-set=?
+  (fn
+    (a b)
+    (and
+      (= (len a) (len b))
+      (dl-at-subset? a b)
+      (dl-at-subset? b a))))
+
+(define
+  dl-at-subset?
+  (fn
+    (xs ys)
+    (cond
+      ((= (len xs) 0) true)
+      ((not (dl-at-contains? ys (first xs))) false)
+      (else (dl-at-subset? (rest xs) ys)))))
+
+(define
+  dl-at-contains?
+  (fn
+    (xs target)
+    (cond
+      ((= (len xs) 0) false)
+      ((dl-at-deep=? (first xs) target) true)
+      (else (dl-at-contains? (rest xs) target)))))
+
+(define
+  dl-at-test!
+  (fn
+    (name got expected)
+    (if
+      (dl-at-deep=? got expected)
+      (set! dl-at-pass (+ dl-at-pass 1))
+      (do
+        (set! dl-at-fail (+ dl-at-fail 1))
+        (append!
+          dl-at-failures
+          (str
+            name
+            "\n    expected: " expected
+            "\n    got:      " got))))))
+
+(define
+  dl-at-test-set!
+  (fn
+    (name got expected)
+    (if
+      (dl-at-set=? got expected)
+      (set! dl-at-pass (+ dl-at-pass 1))
+      (do
+        (set! dl-at-fail (+ dl-at-fail 1))
+        (append!
+          dl-at-failures
+          (str
+            name
+            "\n    expected (set): " expected
+            "\n    got:            " got))))))
+
+(define
+  dl-at-throws?
+  (fn
+    (thunk)
+    (let
+      ((threw false))
+      (do
+        (guard
+          (e (#t (set! threw true)))
+          (thunk))
+        threw))))
+
+(define
+  dl-at-run-all!
+  (fn
+    ()
+    (do
+      ;; count
+      (dl-at-test-set! "count siblings"
+        (dl-query
+          (dl-program
+            "parent(p, bob). parent(p, alice). parent(p, charlie).
+             sibling(X, Y) :- parent(P, X), parent(P, Y), !=(X, Y).
+             sib_count(N) :- count(N, S, sibling(bob, S)).")
+          (list (quote sib_count) (quote N)))
+        (list {:N 2}))
+
+      ;; sum
+      (dl-at-test-set! "sum prices"
+        (dl-query
+          (dl-program
+            "price(apple, 5). price(pear, 7). price(plum, 3).
+             total(T) :- sum(T, X, price(F, X)).")
+          (list (quote total) (quote T)))
+        (list {:T 15}))
+
+      ;; min
+      (dl-at-test-set! "min score"
+        (dl-query
+          (dl-program
+            "score(alice, 80). score(bob, 65). score(carol, 92).
+             lo(M) :- min(M, S, score(P, S)).")
+          (list (quote lo) (quote M)))
+        (list {:M 65}))
+
+      ;; max
+      (dl-at-test-set! "max score"
+        (dl-query
+          (dl-program
+            "score(alice, 80). score(bob, 65). score(carol, 92).
+             hi(M) :- max(M, S, score(P, S)).")
+          (list (quote hi) (quote M)))
+        (list {:M 92}))
+
+      ;; count over derived relation (stratification needed).
+      (dl-at-test-set! "count over derived"
+        (dl-query
+          (dl-program
+            "parent(a, b). parent(a, c). parent(b, d). parent(c, e).
+             ancestor(X, Y) :- parent(X, Y).
+             ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).
+             num_ancestors(N) :- count(N, X, ancestor(a, X)).")
+          (list (quote num_ancestors) (quote N)))
+        (list {:N 4}))
+
+      ;; count with no matches → 0.
+      (dl-at-test-set! "count empty"
+        (dl-query
+          (dl-program
+            "p(1). p(2).
+             zero(N) :- count(N, X, q(X)).")
+          (list (quote zero) (quote N)))
+        (list {:N 0}))
+
+      ;; sum with no matches → 0.
+      (dl-at-test-set! "sum empty"
+        (dl-query
+          (dl-program
+            "p(1). p(2).
+             total(T) :- sum(T, X, q(X)).")
+          (list (quote total) (quote T)))
+        (list {:T 0}))
+
+      ;; min with no matches → rule does not fire.
+      (dl-at-test-set! "min empty"
+        (dl-query
+          (dl-program
+            "p(1). p(2).
+             lo(M) :- min(M, X, q(X)).")
+          (list (quote lo) (quote M)))
+        (list))
+
+      ;; Aggregate with comparison filter on result.
+      (dl-at-test-set! "popularity threshold"
+        (dl-query
+          (dl-program
+            "post(p1). post(p2).
+             liked(u1, p1). liked(u2, p1). liked(u3, p1).
+             liked(u1, p2). liked(u2, p2).
+             popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).")
+          (list (quote popular) (quote P)))
+        (list {:P (quote p1)}))
+
+      ;; Aggregate vs single distinct.
+      (dl-at-test-set! "distinct counted once"
+        (dl-query
+          (dl-program
+            "rated(alice, x). rated(alice, y). rated(bob, x).
+             rater_count(N) :- count(N, U, rated(U, F)).")
+          (list (quote rater_count) (quote N)))
+        (list {:N 2})))))
+
+(define
+  dl-aggregates-tests-run!
+  (fn
+    ()
+    (do
+      (set! dl-at-pass 0)
+      (set! dl-at-fail 0)
+      (set! dl-at-failures (list))
+      (dl-at-run-all!)
+      {:passed dl-at-pass
+       :failed dl-at-fail
+       :total (+ dl-at-pass dl-at-fail)
+       :failures dl-at-failures})))

plans/datalog-on-sx.md

@@ -185,12 +185,27 @@ large graphs.
       rejection, and a negation safety violation.
 
 ### Phase 8 — aggregation (Datalog+)
-- [ ] `count(X, Goal)` → number of distinct X satisfying Goal
-- [ ] `sum(X, Goal)` → sum of X values satisfying Goal
-- [ ] `min(X, Goal)` / `max(X, Goal)` → min/max of X satisfying Goal
-- [ ] `group-by` semantics: `count(X, sibling(bob, X))` → count of bob's siblings
-- [ ] Aggregation breaks stratification — evaluate in a separate post-fixpoint pass
-- [ ] Tests: social network statistics, grade aggregation, inventory sums
+- [x] `(count R V Goal)`, `(sum R V Goal)`, `(min R V Goal)`,
+      `(max R V Goal)` — first arg is the result variable, second is the
+      aggregated variable, third is the goal literal. Live in
+      `lib/datalog/aggregates.sx`.
+- [x] `dl-eval-aggregate`: runs `dl-find-bindings` on the goal under the
+      current subst (which provides outer-context bindings), collects
+      distinct values of the aggregated var, applies the aggregate.
+      `count`/`sum` produce 0 when no matches; `min`/`max` produce no
+      binding (rule fails) when empty.
+- [x] Group-by emerges naturally: outer-context vars in the goal are
+      substituted from the current subst, so `popular(P) :- post(P),
+      count(N, U, liked(U, P)), >=(N, 3).` correctly counts per-post.
+- [x] Stratification: `dl-aggregate-dep-edge` returns a negation-like
+      edge so the aggregate's goal relation is fully derived before the
+      aggregate fires. Non-monotonicity respected.
+- [x] Safety: aggregate body lit binds the result var; goal-internal
+      vars are existentially quantified and don't need outer binding.
+- [x] Tests in `lib/datalog/tests/aggregates.sx` (10): count siblings,
+      sum prices, min/max scores, count over derived relation,
+      empty-input cases for each operator, popularity threshold with
+      group-by, distinct-counted-once.
 
 ### Phase 9 — SX embedding API
 - [ ] `(dl-program facts rules)` → database from SX data directly (no parsing required)
@@ -230,6 +245,19 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Phase 8 done. New `lib/datalog/aggregates.sx` (~110
+  LOC): count / sum / min / max. Each is a body literal of shape
+  `(op R V Goal)` — `dl-eval-aggregate` runs `dl-find-bindings` on
+  the goal under the outer subst (so outer vars in the goal get
+  substituted, giving group-by-style aggregation), collects the
+  distinct values of `V`, and binds `R`. Empty input: count/sum
+  return 0; min/max produce no binding (rule fails). Stratifier
+  extended via `dl-aggregate-dep-edge` so the aggregate's goal
+  relation is fully derived before the aggregate fires. Safety check
+  treats goal-internal vars as existentials (no outer binding
+  required); only the result var becomes bound. Conformance now
+  134 / 134.
+
 - 2026-05-08 — Phase 7 done (Phase 6 magic sets deferred — opt-in,
   semi-naive default suffices for current test suite). New
   `lib/datalog/strata.sx` (~290 LOC): dep graph build, Floyd-Warshall