datalog: arith / by zero raises instead of returning inf

`is(R, /(X, 0))` was silently producing IEEE infinity:

  (dl-eval "p(10). q(R) :- p(X), is(R, /(X, 0))." "?- q(R).")
  => ({:R inf})

That value then flowed through comparisons (anything < inf, anything
> inf) and aggregations (sum of inf, max of inf) producing nonsense
results downstream. `dl-eval-arith` now checks the divisor before
the host `/` and raises "division by zero in <expr>" — surfacing
the bug at its source rather than letting infinity propagate.

1 new test; conformance 264/264.

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

lib/datalog/builtins.sx

@@ -72,7 +72,13 @@
                     ((= rel "+") (+ a b))
                     ((= rel "-") (- a b))
                     ((= rel "*") (* a b))
-                    ((= rel "/") (/ a b))
+                    ((= rel "/")
+                      (cond
+                        ((= b 0)
+                          (error
+                            (str "datalog arith: division by zero in "
+                                 w)))
+                        (else (/ a b))))
                     (else (error (str "datalog arith: unknown op " rel)))))))))
         (else (error (str "datalog arith: not a number — " w)))))))
 

lib/datalog/scoreboard.json

@@ -1,14 +1,14 @@
 {
   "lang": "datalog",
-  "total_passed": 263,
+  "total_passed": 264,
   "total_failed": 0,
-  "total": 263,
+  "total": 264,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
     {"name":"unify","passed":29,"failed":0,"total":29},
     {"name":"eval","passed":39,"failed":0,"total":39},
-    {"name":"builtins","passed":23,"failed":0,"total":23},
+    {"name":"builtins","passed":24,"failed":0,"total":24},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":23,"failed":0,"total":23},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:56:45+00:00"
+  "generated": "2026-05-11T07:59:10+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**263 / 263 passing** (0 failure(s)).
+**264 / 264 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -8,7 +8,7 @@
 | parse | 22 | 22 | ok |
 | unify | 29 | 29 | ok |
 | eval | 39 | 39 | ok |
-| builtins | 23 | 23 | ok |
+| builtins | 24 | 24 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
 | aggregates | 23 | 23 | ok |

lib/datalog/tests/builtins.sx

@@ -239,7 +239,17 @@
         "safe — = binds head var"
         (dl-bt-throws?
           (fn () (dl-program "p(a). p(b). x(Y) :- p(X), =(Y, X).")))
-        false))))
+        false)
+
+      ;; Division by zero raises with a clear error. Without this guard
+      ;; SX's `/` returned IEEE infinity, which then silently flowed
+      ;; through comparisons and aggregations.
+      (dl-bt-test!
+        "is — division by zero raises"
+        (dl-bt-throws?
+          (fn ()
+            (dl-eval "p(10). q(R) :- p(X), is(R, /(X, 0))." "?- q(R).")))
+        true))))
 
 (define
   dl-builtins-tests-run!

plans/datalog-on-sx.md

@@ -15,7 +15,7 @@ for rose-ash data (e.g. federation graph, content relationships).
 
 ## Status (rolling)
 
-`bash lib/datalog/conformance.sh` → **263/263 across 11 suites**
+`bash lib/datalog/conformance.sh` → **264/264 across 11 suites**
 (tokenize, parse, unify, eval, builtins, semi_naive, negation, aggregates,
 api, magic, demo). Source is ~3100 LOC, tests ~2900 LOC, public API
 documented in `lib/datalog/datalog.sx`.
@@ -320,6 +320,12 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-11 — Division by zero in `is` silently produced IEEE
+  infinity instead of raising. `is(R, /(X, 0))` returned `R = inf`,
+  which then flowed through comparisons and aggregations to produce
+  nonsense results. `dl-eval-arith` now raises with a clear
+  "division by zero in <expr>" message. 1 new test; 264/264.
+
 - 2026-05-11 — Aggregate variable validation: `count(N, Y, p(X))`
   silently returned `N = 1` because `Y` was never bound in `p(X)` —
   every match contributed the same unbound symbol, which dl-val-member?