Hello, By a rather indirect route, I discovered that I obtain an almost factor of two improvement in performance in Data.List.sort if I make one small change in the implementation of the function merge which supports mergesort and hence sortBy and sort. Admittedly, the improvement was only noticeable to me when sorting for example one million random Int. The current code in libraries/base/Data/List.hs for merge is \begin{code} merge :: (a -> a -> Ordering) -> [a] -> [a] -> [a] merge cmp xs [] = xs merge cmp [] ys = ys merge cmp (x:xs) (y:ys) = case x `cmp` y of GT -> y : merge cmp (x:xs) ys _ -> x : merge cmp xs (y:ys) \end{code} and all that I did was \begin{code} merge :: (a -> a -> Ordering) -> [a] -> [a] -> [a] merge cmp [] ys = ys merge cmp xs [] = xs merge cmp (x:xs) (y:ys) = case x `cmp` y of GT -> y : merge cmp (x:xs) ys _ -> x : merge cmp xs (y:ys) \end{code} that is, I swapped the order of the first two lines. By the way, the second version is much less likely to overflow the stack than the first version. Can some confirm this? If you confirm it, can someone explain to me why one obtains this performance improvement? I currently just do not grasp the point. Thanks, - Marcus -- Marcus D. Gabriel, Ph.D. Email: mdg@wanadoo.fr 213 ter, rue de Mulhouse Tel: +33.3.89.69.05.06 F68300 Saint Louis FRANCE Portable: +33.6.34.56.07.75