From 1f57620df2ff18881007b3bc56fb8ffbe5efbccf Mon Sep 17 00:00:00 2001
From: "Neil Mitchell http://www.cs.york.ac.uk/~ndm/" <none>
Date: Tue, 8 May 2007 14:31:58 +0200
Subject: Change the swap function so its Haskell 98, by using list-comps
 instead of pattern-guards.

darcs-hash:20070508123158-fbc8d-1592375172ef513d85c5e0095c689abf9ac5fc9b
---
 StackSet.hs | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/StackSet.hs b/StackSet.hs
index 3e35f53..7883daa 100644
--- a/StackSet.hs
+++ b/StackSet.hs
@@ -1,5 +1,3 @@
-{-# OPTIONS -fglasgow-exts #-}
-
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  StackSet
@@ -8,7 +6,7 @@
 --
 -- Maintainer  :  dons@cse.unsw.edu.au
 -- Stability   :  stable
--- Portability :  portable, needs GHC 6.6
+-- Portability :  portable
 --
 -----------------------------------------------------------------------------
 --
@@ -201,12 +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 | a == b  = xs    -- do nothing
-            | Just ai <- L.elemIndex a xs
-            , Just bi <- L.elemIndex b xs = insertAt bi a (insertAt ai b xs)
+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
             where (as,bs) = splitAt n ys
-swap _ _ xs = xs -- do nothing
 
 --
 -- cycling:
-- 
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