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

[larry.liuxinyu]

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 <nick.smallb... at gmail.com> wrote:
> "larry.liuxinyu" <liuxiny... at gmail.com> 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
>
> > 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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