diff --git a/core/core.mal b/core/core.mal
index b88fec2..ac47694 100644
--- a/core/core.mal
+++ b/core/core.mal
@@ -161,11 +161,9 @@
     (def! filter-r (fn* [l p]
         (if (empty? l)
             p
-            (do
-                (def! t (car l))
-                (if (f t)
-                    (filter-r (cdr l) (cons t p))
-                    (filter-r (cdr l) p))))))
+            (if (f (def! t (car l)))
+                (filter-r (cdr l) (cons t p))
+                (filter-r (cdr l) p)))))
     (reverse (filter-r l '()))))
 
 (def! map-if (fn* [c f l]
diff --git a/libs/lists.mal b/libs/lists.mal
index 06a6e61..c48a3a7 100644
--- a/libs/lists.mal
+++ b/libs/lists.mal
@@ -13,10 +13,3 @@
         (let* [c (car n) n (cdr n)]
             (distribute-r (cons c p) n (cons (cons c (concat p n)) t))))))
     (distribute-r '() x '())))
-
-(def! len (fn* [l] (def! len-r (fn* [l c]
-    (if (empty? l)
-        c
-        (len-r (cdr l) (+ c 1)))))
-    (len-r l 0)))
-