I have changed your code little bit, and now it works. merge :: Ord a => [a] -> [a] -> [a] merge [] second = second merge first [] = first merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second | otherwise = y : merge first ys The reason your code is not working because merge [4,5] [] is trying to match it against merge [x] [] = [x] which expects one element list at first place so merge [1] [] would work,but not merge (list having more than one element) []. Best, Mukesh Tiwari On Fri, May 18, 2018 at 3:20 PM, trent shipley <trent.shipley@gmail.com> wrote:
The below produces an error. And I am very proud that I could use the GHCi debugging tools to get this far.
merge [] [] works.
merge [1] [] works.
I don't know why the failing example fails. It should return:
[4,5]
Help to unstuck is appreciated.
:step merge [4,5] []
*** Exception: ex6_8.hs:(12,1)-(16,66): Non-exhaustive patterns in function merge
Given:
merge :: Ord a => [a] -> [a] -> [a]
merge [] [] = []
merge [x] [] = [x]
merge [] [y] = [y]
merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second
| otherwise = y : merge first ys
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