From a3b73a28c0f9d6d2473514cd3c646e62f3931789 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Thu, 18 Dec 2003 20:06:04 +0000
Subject: [PATCH] .

original commit: e45c0400a06bae6e850d17b3ca45869807134411
---
 collects/mzlib/list.ss | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/collects/mzlib/list.ss b/collects/mzlib/list.ss
index 9d1ebf0..a49bb2b 100644
--- a/collects/mzlib/list.ss
+++ b/collects/mzlib/list.ss
@@ -257,14 +257,17 @@
        (unless (and (procedure? f)
                     (procedure-arity-includes? f 1))
          (raise-type-error 'filter "procedure (arity 1)" f))
-       (let loop ([l list])
+       ;; We use the reverse! trick because it's too easy to
+       ;;  overflow the internal stack using natural recursion.
+       ;; It's too bad that our Scheme system is so bad, but
+       ;;  until someone fixes it...
+       (let loop ([l list][result null])
          (cond
-	   [(null? l) null]
+	   [(null? l) (reverse! result)]
 	   [(pair? l)
-	    (let* ([keep? (f (car l))])
-	      (if keep?
-		  (cons (car l) (loop (cdr l)))
-		  (loop (cdr l))))]
+	    (loop (cdr l) (if (f (car l))
+			      (cons (car l) result)
+			      result))]
 	   [else (raise-mismatch-error
 		  'filter
 		  "expects a proper list: "