If you compile with -Wall, you get the following foo.hs:2:1: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In an equation for ‘merge’: Patterns not matched: [] (_:_:_) (_:_:_) [] | 2 | merge [] [] = [] | ^^^^^^^^^^^^^^^^... That is to say, you never match if there is an empty list and a list of 2 or more. Try this: merge :: Ord a => [a] -> [a] -> [a] merge [] ys = ys merge xs [] = xs merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second | otherwise = y : merge first ys On Thu, May 17, 2018 at 10:21 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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