commerce: deterministic subtotal + jurisdiction-relational tax (20 tests)

price.sx — cart-subtotal (unit price = base + variant delta, default 0),
taxo facts indexed by (jurisdiction, product-class, customer-class) -> bps
queried both directions, apply-bps half-up integer rounding, cart-total
returning {:subtotal :discounts :tax :total} reproducible from
(context, cart). Total 54/54.

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

lib/commerce/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart)
+SUITES=(catalog cart price)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -45,6 +45,7 @@ run_suite() {
 (load "lib/minikanren/defrel.sx")
 (load "lib/commerce/catalog.sx")
 (load "lib/commerce/cart.sx")
+(load "lib/commerce/price.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")

lib/commerce/price.sx

@@ -0,0 +1,110 @@
+;; lib/commerce/price.sx — deterministic subtotal + jurisdiction-relational tax.
+;;
+;; A pricing context bundles the inputs that make a total reproducible:
+;;   {:catalog CAT :tax-rules RULES :jurisdiction J :customer C}
+;; Same context + same cart => identical total, every run.
+;;
+;; Tax is NOT a hardcoded VAT rate. Rules are facts indexed by
+;;   (jurisdiction, product-class, customer-class) -> rate-bps
+;; where rate-bps is an integer in basis points (2000 = 20%). taxo queries
+;; them multidirectionally. Money stays in integer minor units; rounding is
+;; half-up per line via integer arithmetic only — never floats.
+
+(define
+  make-pricing-context
+  (fn (catalog tax-rules jurisdiction customer) {:customer customer :jurisdiction jurisdiction :catalog catalog :tax-rules tax-rules}))
+
+(define ctx-catalog (fn (ctx) (get ctx :catalog)))
+
+;; --- unit + line pricing ---
+
+;; Variant delta, defaulting to 0 when the (sku,variant) has no variant fact.
+(define
+  variant-delta
+  (fn
+    (cat sku variant)
+    (let
+      ((rs (run 1 d (varianto cat sku variant d))))
+      (if (empty? rs) 0 (first rs)))))
+
+;; Effective unit price = base price + variant delta. nil if sku unknown.
+(define
+  line-unit-price
+  (fn
+    (cat sku variant)
+    (let
+      ((base (catalog-price cat sku)))
+      (if (nil? base) nil (+ base (variant-delta cat sku variant))))))
+
+;; Extended (line) price = unit price * quantity.
+(define
+  line-extended
+  (fn
+    (cat line)
+    (*
+      (line-unit-price cat (line-sku line) (line-variant line))
+      (line-qty line))))
+
+(define
+  cart-subtotal
+  (fn
+    (cat cart)
+    (reduce (fn (acc l) (+ acc (line-extended cat l))) 0 cart)))
+
+;; --- tax (jurisdiction-relational) ---
+
+;; rules: (list (list jurisdiction class customer bps) ...)
+(define
+  taxo
+  (fn
+    (rules juris class cust bps)
+    (membero (list juris class cust bps) rules)))
+
+;; Deterministic rate lookup; 0 when no rule matches.
+(define
+  rate-bps
+  (fn
+    (rules juris class cust)
+    (let
+      ((rs (run 1 b (taxo rules juris class cust b))))
+      (if (empty? rs) 0 (first rs)))))
+
+;; Apply a basis-point rate to an integer amount, rounding half up.
+(define
+  apply-bps
+  (fn (amount bps) (quotient (+ (* amount bps) 5000) 10000)))
+
+(define
+  line-tax
+  (fn
+    (ctx line)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((class (catalog-class cat (line-sku line))))
+        (apply-bps
+          (line-extended cat line)
+          (rate-bps
+            (get ctx :tax-rules)
+            (get ctx :jurisdiction)
+            class
+            (get ctx :customer)))))))
+
+(define
+  cart-tax
+  (fn
+    (ctx cart)
+    (reduce (fn (acc l) (+ acc (line-tax ctx l))) 0 cart)))
+
+;; --- total ---
+
+;; Returns {:subtotal :discounts :tax :total}. discounts is 0 until Phase 2.
+(define
+  cart-total
+  (fn
+    (ctx cart)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((sub (cart-subtotal cat cart)) (tax (cart-tax ctx cart)))
+        {:subtotal sub :discounts 0 :total (+ sub tax) :tax tax}))))

lib/commerce/scoreboard.json

@@ -1,9 +1,10 @@
 {
   "suites": {
     "catalog": {"pass": 16, "fail": 0},
-    "cart": {"pass": 18, "fail": 0}
+    "cart": {"pass": 18, "fail": 0},
+    "price": {"pass": 20, "fail": 0}
   },
-  "total_pass": 34,
+  "total_pass": 54,
   "total_fail": 0,
-  "total": 34
+  "total": 54
 }

lib/commerce/scoreboard.md

@@ -6,4 +6,5 @@ _Generated by `lib/commerce/conformance.sh`_
 |-------|-----:|-----:|------:|
 | catalog | 16 | 0 | 16 |
 | cart | 18 | 0 | 18 |
-| **Total** | **34** | **0** | **34** |
+| price | 20 | 0 | 20 |
+| **Total** | **54** | **0** | **54** |

lib/commerce/tests/price.sx

@@ -0,0 +1,100 @@
+;; lib/commerce/tests/price.sx — subtotal + jurisdiction-relational tax.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+(define
+  pcat
+  (make-catalog
+    (list
+      (list "widget" 1000 :standard)
+      (list "book" 800 :zero-rated)
+      (list "tea" 1000 :reduced))
+    (list
+      (list "widget" :small -200)
+      (list "widget" :large 500))
+    (list)))
+
+(define
+  rules
+  (list
+    (list :uk :standard :guest 2000)
+    (list :uk :reduced :guest 500)
+    (list :uk :zero-rated :guest 0)
+    (list :uk :standard :member 1000)
+    (list :ie :standard :guest 2300)))
+
+(define gctx (make-pricing-context pcat rules :uk :guest))
+(define mctx (make-pricing-context pcat rules :uk :member))
+
+;; --- unit + line pricing ---
+
+(commerce-test
+  "unit-price-variant"
+  (line-unit-price pcat "widget" :small)
+  800)
+(commerce-test
+  "unit-price-no-variant"
+  (line-unit-price pcat "widget" :none)
+  1000)
+(commerce-test "unit-price-unknown" (line-unit-price pcat "ghost" :none) nil)
+(commerce-test
+  "line-extended"
+  (line-extended pcat (list "widget" :small 2))
+  1600)
+
+;; --- subtotal ---
+
+(define
+  cart1
+  (list (list "widget" :small 2) (list "book" :none 1)))
+
+(commerce-test "subtotal" (cart-subtotal pcat cart1) 2400)
+(commerce-test "subtotal-empty" (cart-subtotal pcat empty-cart) 0)
+
+;; --- tax rate lookup (relational, both directions) ---
+
+(commerce-test
+  "rate-forward"
+  (rate-bps rules :uk :standard :guest)
+  2000)
+(commerce-test
+  "rate-missing"
+  (rate-bps rules :fr :standard :guest)
+  0)
+(commerce-test
+  "rate-juris-by-bps-backward"
+  (run* j (fresh (cust) (taxo rules j :standard cust 2300)))
+  (list :ie))
+(commerce-test
+  "rate-customer-by-bps-backward"
+  (run* cust (taxo rules :uk :standard cust 1000))
+  (list :member))
+
+;; --- apply-bps rounding (half up, integer only) ---
+
+(commerce-test "bps-exact" (apply-bps 1600 2000) 320)
+(commerce-test "bps-round-up" (apply-bps 799 2000) 160)
+(commerce-test "bps-zero" (apply-bps 800 0) 0)
+
+;; --- line + cart tax ---
+
+(commerce-test
+  "line-tax-standard"
+  (line-tax gctx (list "widget" :small 2))
+  320)
+(commerce-test
+  "line-tax-zero-rated"
+  (line-tax gctx (list "book" :none 1))
+  0)
+(commerce-test
+  "line-tax-member"
+  (line-tax mctx (list "widget" :small 2))
+  160)
+(commerce-test "cart-tax-guest" (cart-tax gctx cart1) 320)
+
+;; --- total dict (deterministic) ---
+
+(commerce-test "total-guest" (cart-total gctx cart1) {:subtotal 2400 :discounts 0 :total 2720 :tax 320})
+
+(commerce-test "total-member" (cart-total mctx cart1) {:subtotal 2400 :discounts 0 :total 2560 :tax 160})
+
+(commerce-test "total-empty" (cart-total gctx empty-cart) {:subtotal 0 :discounts 0 :total 0 :tax 0})

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **34/34** (2 suites: catalog, cart)
+`bash lib/commerce/conformance.sh` → **54/54** (3 suites: catalog, cart, price)
 
 ## Ground rules
 
@@ -57,7 +57,7 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 ## Phase 1 — Catalog + cart + deterministic totals
 - [x] `catalog.sx` — product/variant/stock as facts
 - [x] `cart.sx` — line items, add/remove/qty
-- [ ] `price.sx` — base pricing relation, subtotal; tax
+- [x] `price.sx` — base pricing relation, subtotal; tax
 - [ ] `api.sx` + tests + scoreboard + conformance.sh
 
 ## Phase 2 — Promotions (relational)
@@ -76,6 +76,13 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [ ] tests: webhook replay, partial refund, double-charge guard
 
 ## Progress log
+- 2026-06-06 — `price.sx`: deterministic `cart-subtotal` (Σ unit×qty, variant
+  delta defaults 0) + jurisdiction-relational tax. `taxo` facts indexed by
+  (jurisdiction, product-class, customer-class)→bps, queried multidirectionally;
+  `apply-bps` rounds half-up with integer arithmetic only. `cart-total` returns
+  `{:subtotal :discounts :tax :total}` (discounts 0 until Phase 2), reproducible
+  from (context, cart). `=` does structural dict equality (order-independent), so
+  total dicts compare directly. price suite 20/20; total 54/54.
 - 2026-06-06 — `cart.sx`: cart as an ordered list of (sku variant qty) lines.
   Pure ops `cart-add` (merges same line / appends), `cart-set-qty` (0 removes),
   `cart-remove`, plus `cart-qty`/`cart-count`/`cart-skus`/`cart-empty?`.