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-rw-r--r--StackSet.hs9
1 files 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
--