gistfile1.txt

import qualified Data.List.NonEmpty as NE
import           Test.QuickCheck

main :: IO ()
main = do
  quickCheck prop_biggest

prop_biggest :: NE.NonEmpty Int -> Bool
prop_biggest xs =
  biggest xs == (NE.last . NE.sort $ xs)

biggest :: NE.NonEmpty Int -> Int
biggest = NE.head . (NE.sortWith id)

gistfile2.txt

test/Main.hs:6:3: error:
    • No instance for (Arbitrary (NE.NonEmpty Int))
        arising from a use of ‘quickCheck’
    • In a stmt of a 'do' block: quickCheck prop_biggest
      In the expression: do quickCheck prop_biggest
      In an equation for ‘main’: main = do quickCheck prop_biggest
  |
6 |   quickCheck prop_biggest
  |   ^^^^^^^^^^^^^^^^^^^^^^^