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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