如何将合并列表按升序排序？

Question

merge :: [a] -> [a] -> [a]

 merge xs     []     = xs
 merge []     ys     = ys
 merge (x:xs) (y:ys) = x : y : merge xs ys

I have got this working but now I need to sort them in ascending order.

Answer 1

You just need to compare x and y and decide which to add to the result first. Note that you only add one at a time; the next element after x might still come before y .

Note you need the Ord constraint to ensure that <= is defined for a .

merge :: Ord a => [a] -> [a] -> [a]
merge xs [] = xs
merge [] ys = ys
merge xs@(x:xs') ys@(y:ys') | x <= y = x : merge xs' ys
                            | otherwise = y : merge xs ys'