mergesort

sort.hs

mergesort :: (a -> a -> Ordering) -> [a] -> [a]
mergesort cmp = mergesort' cmp . map wrap

mergesort' :: (a -> a -> Ordering) -> [[a]] -> [a]
mergesort' _   [] = []
mergesort' _   [xs] = xs
mergesort' cmp xss = mergesort' cmp (merge_pairs cmp xss)

merge_pairs :: (a -> a -> Ordering) -> [[a]] -> [[a]]
merge_pairs _   [] = []
merge_pairs _   [xs] = [xs]
merge_pairs cmp (xs:ys:xss) = merge cmp xs ys : merge_pairs cmp xss

merge :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
merge _   [] ys = ys
merge _   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)

wrap :: a -> [a]
wrap x = [x]