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-rw-r--r--StackSet.hs3
1 files changed, 2 insertions, 1 deletions
diff --git a/StackSet.hs b/StackSet.hs
index 38baca2..e0210f1 100644
--- a/StackSet.hs
+++ b/StackSet.hs
@@ -188,7 +188,8 @@ raiseFocus k w = case M.lookup k (cache w) of
promote :: (Integral i, Ord a) => StackSet i j a -> StackSet i j a
promote w = maybe w id $ do
a <- peek w -- fail if null
- let w' = w { stacks = M.adjust (\(f, s) -> (f, swap a (head s) s)) (current w) (stacks w) }
+ (f, xs@(x:_)) <- M.lookup (current w) (stacks w)
+ let w' = w { stacks = M.insert (current w) (f, swap a x xs) (stacks w) }
return $ insert a (current w) w' -- and maintain focus (?)
-- | Swap first occurences of 'a' and 'b' in list.