Hi, Thanks a lot. The following type protocol also works. prop_foo :: (Ord a)=>(Num a) => [a] -> Property Somebody told me that: Eduard Sergeev • BTW, more recent QuickCheck (from Haskell Platform 2011.2.0.X - contains QuickCheck-2.4.0.1) seems to identifies the problem correctly: *** Failed! Falsifiable (after 3 tests and 2 shrinks): [0,1] False -- Larry On Apr 20, 11:36 pm, Nick Smallbone wrote:

"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-ev... 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

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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" :)

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On Thu, 2011-04-21 at 23:56 +0200, Nick Smallbone wrote:

"larry.liuxinyu" writes:

...

Somebody told me that: Eduard Sergeev • BTW, more recent QuickCheck (from Haskell Platform 2011.2.0.X - contains QuickCheck-2.4.0.1) seems to identifies the problem correctly:

*** Failed! Falsifiable (after 3 tests and 2 shrinks): [0,1] False

I don't think this can be true: the problem occurs in GHCi and there's no way for QuickCheck to detect it. And when I tested it I got the same problem. There must be some difference between the properties you both tested...

Nick

There was an additional class contrain (Num a). Num default to Int IIRC. Regards

Hi, I tested with Haskell platform 2011 with QuickCheck 2.4.0.1. It produced 100 cases passed, but can't report failed case. verboseCheck still told me that [(), (), ... ()] are generated as instance to (Ord a) The only way is to specify the non-ambitious type for example, Int, like below: test (prop_foo::[Int]->Property) Cheers. -- Larry. On Apr 22, 5:56 am, Nick Smallbone wrote:

"larry.liuxinyu" writes:

...

Somebody told me that: Eduard Sergeev • BTW, more recent QuickCheck (from Haskell Platform 2011.2.0.X - contains QuickCheck-2.4.0.1) seems to identifies the problem correctly:

...

*** Failed! Falsifiable (after 3 tests and 2 shrinks): [0,1] False

I don't think this can be true: the problem occurs in GHCi and there's no way for QuickCheck to detect it. And when I tested it I got the same problem. There must be some difference between the properties you both tested...

Nick

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