fix interleave not fully lazy

1 files changed, 12 insertions, 4 deletions

diff --git a/src/clj/clojure/core.clj b/src/clj/clojure/core.clj
index 4096a6ff..ef8b40e2 100644
--- a/src/clj/clojure/core.clj
+++ b/src/clj/clojure/core.clj
@@ -2588,10 +2588,18 @@
        (cons (first s) (take-nth n (drop n s))))))
 
 (defn interleave
-  "Returns a lazy seq of the first item in each coll, then the second
-  etc."
-  [& colls]
-    (apply concat (apply map list colls)))
+  "Returns a lazy seq of the first item in each coll, then the second etc."
+  ([c1 c2]
+     (lazy-seq
+      (let [s1 (seq c1) s2 (seq c2)]
+        (when (and s1 s2)
+          (cons (first s1) (cons (first s2) 
+                                 (interleave (rest s1) (rest s2))))))))
+  ([c1 c2 & colls] 
+     (lazy-seq 
+      (let [ss (map seq (conj colls c2 c1))]
+        (when (every? identity ss)
+          (concat (map first ss) (apply interleave (map rest ss))))))))
 
 (defn var-get
   "Gets the value in the var object"