mod: Ext 10 — policy what-if / impact analysis, 260/260

mod/decision-diff compares one report's action under two rule sets;
mod/policy-impact batches a set and returns only the reports whose decision flips;
mod/impact-count / mod/impact-report summarize. Lets a mod team measure a policy
change's blast radius before shipping (e.g. removing spam-hide flips r1 hide→keep).
Pure SX over decide-report. +13 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/mod/tests/whatif.sx

@@ -0,0 +1,117 @@
+;; lib/mod/tests/whatif.sx — Ext 10: policy what-if / impact analysis.
+
+(define mod-wi-count 0)
+(define mod-wi-pass 0)
+(define mod-wi-fail 0)
+(define mod-wi-failures (list))
+
+(define
+  mod-wi-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! mod-wi-count (+ mod-wi-count 1))
+      (if
+        (= got expected)
+        (set! mod-wi-pass (+ mod-wi-pass 1))
+        (begin
+          (set! mod-wi-fail (+ mod-wi-fail 1))
+          (append!
+            mod-wi-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+;; rules-b is the default policy with spam-hide removed: spam now falls through
+;; to default-keep. A spam report flips hide → keep; everything else is unchanged.
+(define mod-wi-rules-a mod/default-rules)
+(define
+  mod-wi-rules-b
+  (list
+    (mod/mk-rule
+      "reviewer-remove"
+      :remove (list (list :evidence "confirmed-abuse")))
+    (mod/mk-rule
+      "abuse-remove"
+      :remove (list (list :classification "abuse")))
+    (mod/mk-rule
+      "repeated-escalate"
+      :escalate (list (list :count-at-least 3)))
+    (mod/mk-rule "default-keep" :keep (list))))
+
+(define mod-wi-spam (mod/mk-report "r1" "a" "bob" "this is spam"))
+(define mod-wi-abuse (mod/mk-report "r2" "a" "carol" "harassment here"))
+(define mod-wi-clean (mod/mk-report "r3" "a" "dave" "a fine post"))
+
+;; ── single-report diff ──
+
+(define
+  mod-wi-d
+  (mod/decision-diff
+    mod-wi-spam
+    (list mod-wi-spam)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "spam before = hide" (get mod-wi-d :before) "hide")
+(mod-wi-test! "spam after = keep" (get mod-wi-d :after) "keep")
+(mod-wi-test! "spam decision flips" (get mod-wi-d :changed) true)
+(mod-wi-test! "diff carries report id" (get mod-wi-d :report-id) "r1")
+
+(define
+  mod-wi-da
+  (mod/decision-diff
+    mod-wi-abuse
+    (list mod-wi-abuse)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "abuse unchanged (remove both)" (get mod-wi-da :changed) false)
+(mod-wi-test! "abuse stays remove" (get mod-wi-da :after) "remove")
+
+(define
+  mod-wi-dc
+  (mod/decision-diff
+    mod-wi-clean
+    (list mod-wi-clean)
+    mod-wi-rules-a
+    mod-wi-rules-b))
+(mod-wi-test! "clean unchanged (keep both)" (get mod-wi-dc :changed) false)
+
+;; ── batch impact ──
+
+(define mod-wi-batch (list mod-wi-spam mod-wi-abuse mod-wi-clean))
+(define
+  mod-wi-impact
+  (mod/policy-impact mod-wi-batch mod-wi-rules-a mod-wi-rules-b))
+
+(mod-wi-test!
+  "impact lists only changed reports"
+  (len mod-wi-impact)
+  1)
+(mod-wi-test!
+  "impacted report is the spam one"
+  (get (first mod-wi-impact) :report-id)
+  "r1")
+(mod-wi-test!
+  "impact-count agrees"
+  (mod/impact-count mod-wi-batch mod-wi-rules-a mod-wi-rules-b)
+  1)
+
+;; ── identical rule sets → no impact ──
+
+(mod-wi-test!
+  "same rules → zero impact"
+  (mod/impact-count mod-wi-batch mod-wi-rules-a mod-wi-rules-a)
+  0)
+(mod-wi-test!
+  "same rules → empty report"
+  (mod/impact-report mod-wi-batch mod-wi-rules-a mod-wi-rules-a)
+  "No decisions change.")
+
+;; ── rendering ──
+
+(mod-wi-test!
+  "impact-report renders the flip"
+  (mod/str-contains?
+    (mod/impact-report mod-wi-batch mod-wi-rules-a mod-wi-rules-b)
+    "r1: hide → keep")
+  true)
+
+(define mod-whatif-tests-run! (fn () {:failures mod-wi-failures :total mod-wi-count :passed mod-wi-pass :failed mod-wi-fail}))