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...@gmail.com> wrote: > "larry.liuxinyu" <liuxiny...@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" :) > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
