Hi, today, I want to ask about QuickCheck. I have known recently that arbitrary works on functions as well. I played on this for a while, and met a strange behavior. I tried to write property for sortBy. import Test.QuickCheck import Data.List instance Show (a -> b) where show _ = "<<function>>" instance Arbitrary Ordering where arbitrary = elements [LT,EQ,GT] mySortBy :: (Ord a) => (a -> a -> Ordering) -> [a] -> [a] mySortBy = sortBy prop_mysortby_sorted :: (Int -> Int -> Ordering) -> [Int] -> Property prop_mysortby_sorted cmp ls = null ls `trivial` all eq_or_lt (zipWith cmp sorted (tail sorted)) where sorted = mySortBy cmp ls eq_or_lt ord = ord == EQ || ord == LT I had thought Arbitrary instance for Ord and property for sortBy both fine. But checking fails: ghci> quickCheck prop_mysortby_sorted Falsifiable, after 2 tests: <<function>> [2,3] I guess arbitrary for (Int -> Int -> Ordering) generates a non-sense function like this: -- let (arb_compare :: Int -> Int -> Ordering) generated function. arb_compare 0 1 = GT arb_compare 1 0 = GT arb_compare _ _ = EQ Then, I want to ask two questions. 1. Is my guessing in function generated by arbitrary right? 2. If so, How do I generate right function? Thanks, Yusaku Hashimoto