[Haskell-cafe] QuickCheck, (Ord a)=> [a] -> Property problem

[larry.liuxinyu]

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