From 5709a2f332a3c5fb1749bb94334c92d0ab683397 Mon Sep 17 00:00:00 2001
From: Spencer Janssen <sjanssen@cse.unl.edu>
Date: Tue, 8 May 2007 17:09:43 +0200
Subject: Use 'drop 1' rather than tail, skip equality check.

darcs-hash:20070508150943-a5988-84c0ebc06bc745ecdacae2ced6863b63a9585756
---
 StackSet.hs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/StackSet.hs b/StackSet.hs
index 74d63eb..3f615bf 100644
--- a/StackSet.hs
+++ b/StackSet.hs
@@ -199,10 +199,11 @@ promote w = maybe w id $ do
 -- > swap a b . swap a b == id
 --
 swap :: Eq a => a -> a -> [a] -> [a]
-swap a b xs = head $ [insertAt bi a (insertAt ai b xs) | a /= b
-                     ,Just ai <- [L.elemIndex a xs], Just bi <- [L.elemIndex b xs]]
-                     ++ [xs]
-  where insertAt n x ys = as ++ x : tail bs
+swap a b xs = maybe xs id $ do
+    ai <- L.elemIndex a xs
+    bi <- L.elemIndex b xs
+    return . insertAt bi a . insertAt ai b $ xs
+  where insertAt n x ys = as ++ x : drop 1 bs
             where (as,bs) = splitAt n ys
 
 --
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