erlang: lists zip/2 + zipwith/3 + unzip/1 (841/841)

Zip family in lib/erlang/lists-ext.sx; length mismatch and malformed
pairs raise badarg. lists_ext suite 62 -> 70.

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

lib/erlang/lists-ext.sx

@@ -366,6 +366,62 @@
           (fn (a b) (er-ext-lt? b a)))
         (raise (er-mk-error-marker (er-mk-atom "badarg")))))))
 
+;; ── zip family (zip / zipwith / unzip) ────────────────────────────
+;; Length mismatch raises badarg (real Erlang raises function_clause;
+;; badarg is the closest in-port equivalent).
+(define
+  er-ext-zip
+  (fn (a b)
+    (cond
+      (and (er-nil? a) (er-nil? b)) (er-mk-nil)
+      (and (er-cons? a) (er-cons? b))
+        (er-mk-cons
+          (er-mk-tuple (list (get a :head) (get b :head)))
+          (er-ext-zip (get a :tail) (get b :tail)))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-zip
+  (fn (vs) (er-ext-zip (nth vs 0) (nth vs 1))))
+
+(define
+  er-ext-zipwith
+  (fn (f a b)
+    (cond
+      (and (er-nil? a) (er-nil? b)) (er-mk-nil)
+      (and (er-cons? a) (er-cons? b))
+        (er-mk-cons
+          (er-apply-fun f (list (get a :head) (get b :head)))
+          (er-ext-zipwith f (get a :tail) (get b :tail)))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-zipwith
+  (fn (vs) (er-ext-zipwith (nth vs 0) (nth vs 1) (nth vs 2))))
+
+(define
+  er-ext-unzip
+  (fn (lst as bs)
+    (cond
+      (er-nil? lst)
+        (er-mk-tuple
+          (list
+            (er-list-reverse-iter as (er-mk-nil))
+            (er-list-reverse-iter bs (er-mk-nil))))
+      (and (er-cons? lst) (er-tuple? (get lst :head)))
+        (let ((es (get (get lst :head) :elements)))
+          (if (= (len es) 2)
+            (er-ext-unzip (get lst :tail)
+              (er-mk-cons (nth es 0) as)
+              (er-mk-cons (nth es 1) bs))
+            (raise (er-mk-error-marker (er-mk-atom "badarg")))))
+      :else (raise (er-mk-error-marker (er-mk-atom "badarg"))))))
+
+(define
+  er-bif-lists-unzip
+  (fn (vs)
+    (er-ext-unzip (er-bif-arg1 vs "lists:unzip") (er-mk-nil) (er-mk-nil))))
+
 ;; ── register ──────────────────────────────────────────────────────
 ;; Hook into er-register-builtin-bifs! rather than registering once:
 ;; the registry can be reset + rebuilt mid-run (tests/runtime.sx does
@@ -390,7 +446,10 @@
     (er-register-pure-bif! "lists" "splitwith" 2 er-bif-lists-splitwith)
     (er-register-pure-bif! "lists" "flatten" 1 er-bif-lists-flatten)
     (er-register-pure-bif! "lists" "max" 1 er-bif-lists-max)
-    (er-register-pure-bif! "lists" "min" 1 er-bif-lists-min)))
+    (er-register-pure-bif! "lists" "min" 1 er-bif-lists-min)
+    (er-register-pure-bif! "lists" "zip" 2 er-bif-lists-zip)
+    (er-register-pure-bif! "lists" "zipwith" 3 er-bif-lists-zipwith)
+    (er-register-pure-bif! "lists" "unzip" 1 er-bif-lists-unzip)))
 
 (define er-ext-prev-register-builtins er-register-builtin-bifs!)
 (define er-register-builtin-bifs!