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

"larry.liuxinyu" writes:

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.

This is an unfortunate feature of GHCi: if the thing you want to evaluate has a polymorphic type then all the type variables default to (), see: http://www.haskell.org/ghc/docs/7.0.3/html/users_guide/interactive-evaluatio... So prop_foo is only tested for lists of (). Nasty. The usual way to work around it is to declare all your properties monomorphic, so write: prop_foo :: [Integer] -> Property

This works at least, However, since 'a'<'b', they are order-able, what if I want to test prop_foo works for char?

Testing with Integers should always[*] be enough because of parametricity. Nick [*] For certain values of "always" :)