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

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:

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prop_foo :: (Ord a) => [a] -> Property prop_foo xs = not (null xs) ==> maximum xs == minimum xs

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This is an extreme case that the property is always wrong.

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However, QuickCheck produces: *Main> test prop_foo OK, passed 100 tests.

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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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