commerce: cart line items + add/remove/set-qty + relational view (18 tests)

cart.sx — cart as an ordered list of (sku variant qty) lines. Pure
operations: cart-add (merge-or-append), cart-set-qty (0 removes),
cart-remove, with cart-qty/count/skus/empty? accessors. cart-lineo
exposes lines relationally via membero. Total 34/34.

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

lib/commerce/cart.sx

@@ -0,0 +1,86 @@
+;; lib/commerce/cart.sx — cart as an ordered list of line items.
+;;
+;; A cart is a native list of lines; a line is (list sku variant qty).
+;; All operations are pure: they return a new cart, never mutate. Line
+;; order is insertion order (stable) so totals are reproducible.
+;;
+;; cart-lineo is the relational view — because a line *is* a (sku variant qty)
+;; tuple, membero queries the cart directly, forward or backward.
+
+(define empty-cart (list))
+
+(define make-line (fn (sku variant qty) (list sku variant qty)))
+(define line-sku (fn (l) (nth l 0)))
+(define line-variant (fn (l) (nth l 1)))
+(define line-qty (fn (l) (nth l 2)))
+
+(define
+  same-line?
+  (fn
+    (l sku variant)
+    (and (= (line-sku l) sku) (= (line-variant l) variant))))
+
+(define
+  cart-qty
+  (fn
+    (cart sku variant)
+    (let
+      ((m (filter (fn (l) (same-line? l sku variant)) cart)))
+      (if (empty? m) 0 (line-qty (first m))))))
+
+(define
+  cart-remove
+  (fn
+    (cart sku variant)
+    (filter (fn (l) (not (same-line? l sku variant))) cart)))
+
+;; Add qty units; merges into an existing (sku,variant) line in place,
+;; otherwise appends a new line at the end.
+(define
+  cart-add
+  (fn
+    (cart sku variant qty)
+    (let
+      ((existing (cart-qty cart sku variant)))
+      (if
+        (= existing 0)
+        (append cart (list (make-line sku variant qty)))
+        (map
+          (fn
+            (l)
+            (if
+              (same-line? l sku variant)
+              (make-line sku variant (+ existing qty))
+              l))
+          cart)))))
+
+;; Set the absolute quantity; qty <= 0 removes the line.
+(define
+  cart-set-qty
+  (fn
+    (cart sku variant qty)
+    (if
+      (<= qty 0)
+      (cart-remove cart sku variant)
+      (if
+        (= (cart-qty cart sku variant) 0)
+        (append cart (list (make-line sku variant qty)))
+        (map
+          (fn
+            (l)
+            (if (same-line? l sku variant) (make-line sku variant qty) l))
+          cart)))))
+
+(define cart-empty? (fn (cart) (empty? cart)))
+(define cart-lines (fn (cart) cart))
+(define cart-skus (fn (cart) (map line-sku cart)))
+
+;; Total number of units across all lines.
+(define
+  cart-count
+  (fn (cart) (reduce (fn (acc l) (+ acc (line-qty l))) 0 cart)))
+
+;; Relational view of cart lines.
+(define
+  cart-lineo
+  (fn (cart sku variant qty) (membero (list sku variant qty) cart)))

lib/commerce/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog)
+SUITES=(catalog cart)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -44,6 +44,7 @@ run_suite() {
 (load "lib/minikanren/matche.sx")
 (load "lib/minikanren/defrel.sx")
 (load "lib/commerce/catalog.sx")
+(load "lib/commerce/cart.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")

lib/commerce/scoreboard.json

@@ -1,8 +1,9 @@
 {
   "suites": {
-    "catalog": {"pass": 16, "fail": 0}
+    "catalog": {"pass": 16, "fail": 0},
+    "cart": {"pass": 18, "fail": 0}
   },
-  "total_pass": 16,
+  "total_pass": 34,
   "total_fail": 0,
-  "total": 16
+  "total": 34
 }

lib/commerce/scoreboard.md

@@ -5,4 +5,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | Suite | Pass | Fail | Total |
 |-------|-----:|-----:|------:|
 | catalog | 16 | 0 | 16 |
-| **Total** | **16** | **0** | **16** |
+| cart | 18 | 0 | 18 |
+| **Total** | **34** | **0** | **34** |

lib/commerce/tests/cart.sx

@@ -0,0 +1,103 @@
+;; lib/commerce/tests/cart.sx — cart structure + line operations.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+;; --- add ---
+
+(commerce-test
+  "add-to-empty"
+  (cart-add empty-cart "widget" :small 2)
+  (list (list "widget" :small 2)))
+
+(commerce-test
+  "add-merges-same-line"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "widget"
+    :small 3)
+  (list (list "widget" :small 5)))
+
+(commerce-test
+  "add-different-variant-separate"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "widget"
+    :large 1)
+  (list (list "widget" :small 2) (list "widget" :large 1)))
+
+(commerce-test
+  "add-different-sku-separate"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "gadget"
+    :std 1)
+  (list (list "widget" :small 2) (list "gadget" :std 1)))
+
+(commerce-test
+  "add-preserves-order"
+  (cart-skus
+    (cart-add
+      (cart-add (cart-add empty-cart "a" :v 1) "b" :v 1)
+      "c"
+      :v 1))
+  (list "a" "b" "c"))
+
+;; --- qty queries ---
+
+(define
+  c2
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "gadget"
+    :std 4))
+
+(commerce-test "cart-qty-found" (cart-qty c2 "widget" :small) 2)
+(commerce-test "cart-qty-missing" (cart-qty c2 "widget" :large) 0)
+(commerce-test "cart-count" (cart-count c2) 6)
+(commerce-test "cart-empty-yes" (cart-empty? empty-cart) true)
+(commerce-test "cart-empty-no" (cart-empty? c2) false)
+
+;; --- set-qty ---
+
+(commerce-test
+  "set-qty-existing"
+  (cart-set-qty c2 "widget" :small 10)
+  (list (list "widget" :small 10) (list "gadget" :std 4)))
+
+(commerce-test
+  "set-qty-new-line"
+  (cart-set-qty empty-cart "book" :std 3)
+  (list (list "book" :std 3)))
+
+(commerce-test
+  "set-qty-zero-removes"
+  (cart-set-qty c2 "widget" :small 0)
+  (list (list "gadget" :std 4)))
+
+;; --- remove ---
+
+(commerce-test
+  "remove-line"
+  (cart-remove c2 "gadget" :std)
+  (list (list "widget" :small 2)))
+
+(commerce-test
+  "remove-missing-noop"
+  (cart-remove c2 "nope" :std)
+  (list (list "widget" :small 2) (list "gadget" :std 4)))
+
+;; --- relational view ---
+
+(commerce-test
+  "cart-lineo-forward"
+  (run* q (cart-lineo c2 "gadget" :std q))
+  (list 4))
+
+(commerce-test
+  "cart-lineo-sku-by-qty-backward"
+  (run* sk (fresh (v) (cart-lineo c2 sk v 4)))
+  (list "gadget"))
+
+(commerce-test
+  "cart-lineo-all-skus"
+  (run* sk (fresh (v q) (cart-lineo c2 sk v q)))
+  (list "widget" "gadget"))

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **16/16** (1 suite: catalog)
+`bash lib/commerce/conformance.sh` → **34/34** (2 suites: catalog, cart)
 
 ## Ground rules
 
@@ -56,7 +56,7 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 
 ## Phase 1 — Catalog + cart + deterministic totals
 - [x] `catalog.sx` — product/variant/stock as facts
-- [ ] `cart.sx` — line items, add/remove/qty
+- [x] `cart.sx` — line items, add/remove/qty
 - [ ] `price.sx` — base pricing relation, subtotal; tax
 - [ ] `api.sx` + tests + scoreboard + conformance.sh
 
@@ -76,6 +76,11 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [ ] tests: webhook replay, partial refund, double-charge guard
 
 ## Progress log
+- 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?`.
+  `cart-lineo` is the relational view (membero over the cart) — forward and
+  backward. cart suite 18/18; total 34/34.
 - 2026-06-06 — `catalog.sx`: catalog snapshot (products/variants/stock as fact
   tuples) + multidirectional accessor relations (`producto`/`varianto`/`stocko`,
   derived `priceo`/`classo`/`unit-priceo`) + deterministic `catalog-price`/