Hi Trent, 

This works: 

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

Difference in the lines

merge x [] = x

merge [] y = y

As the input is of type [a] where a belongs to typeclass Ord, you must pass x instead of [x]. 

[x] would work if you tried merge [4] []. but will fail if you tried merge [4,5] []. because "4,5" isn't of type a. 

Regards, Hemanth

On Fri, May 18, 2018 at 10:51 AM 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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