Hi, I found there is similar question as: http://groups.google.com/group/haskell-cafe/browse_thread/thread/7439262e9ac80dd2/91ca18e11ff00649?lnk=gst&q=QuickCheck+Ord+a#91ca18e11ff00649 I am still think it's very strange. For example: prop_foo :: (Ord a) => [a] -> Property prop_foo xs = not (null xs) ==> maximum xs == minimum xs This is an extreme case that the property is always wrong. However, QuickCheck produces: *Main> test prop_foo OK, passed 100 tests. Why this happen? If I use verboseCheck, I can find the sample test data are as the following: *Main>verboseCheck prop_foo ... 97: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),()] 98: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),(),(),(),(),()] 99: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),()] OK, passed 100 tests. So Arbitrary () is generated as the instance of Ord a. I have to change the prototype as prop_foo :: (Num a) => [a] -> Property This works at least, However, since 'a'<'b', they are order-able, what if I want to test prop_foo works for char? I am using Haskell Platform (version 6.10.4), with QuickCheck version 1.2.0.0 Thanks -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/home