점점점과 빈 리스트는 맞음

src/expand/syntax_rules.clj

@@ -68,6 +68,18 @@
   (or (= '... x)
       (and (scm/syntax? x) (-> x scm/syntax-e ellipsis?))))
 
+(defn pattern-vars [pattern]
+  ;; 안 예쁘는데ㅠㅠ
+  (let [vs (atom #{})]
+    (walk/prewalk (fn [x]
+                    (when (and (scm/identifier? x)
+                               (not (wildcard? x))
+                               (not (ellipsis? x)))
+                      (swap! vs #(conj % x)))
+                    x)
+                  pattern)
+    @vs))
+
 (defn unify [pattern datum]
   (match [(scm/syntax-e pattern) (scm/syntax-e datum)]
     [(_ :guard wildcard?) _] {}
@@ -79,13 +91,22 @@
 
     [([p (_ :guard ellipsis?) & ps] :seq)
      (ds :guard seq?)]
-    (let [ds-end (take-last (count ps) ds)
-          ds-start (drop-last (count ps) ds)]
+    (let [ds-end (or (take-last (count ps) ds)
+                     (list))
+          ds-start (or (drop-last (count ps) ds)
+                       (list))]
       (merge-unify
        (some->> ds
                 (map #(some-> (unify p %)
                               (update-vals list)))
-                (apply merge-unify-with concat)
+                (apply merge-unify-with concat
+                       ;; 여기에 이 map이 없으면 ds 빌 때 `unify`는
+                       ;; {}을 생성할 잘못되게 것이다. 그때
+                       ;; variable들은 bound을 안 될 것이다. 대신에
+                       ;; empty ellipsis로 bind해야 한다.
+                       (into {}
+                             (for [v (pattern-vars p)]
+                               [(scm/syntax-e v) (list)])))
                 (map (fn [[k v]]
                        [k [:ellipsis v]]))
                 (into {}))))