[Haskell-cafe] Help using QuickCheck

[Benjamin Franksen]

On Friday 21 October 2005 20:24, Andrew Pimlott wrote:
> On Fri, Oct 21, 2005 at 07:58:10PM +0200, Benjamin Franksen wrote:
> > I am asking help with using QuickCheck. I tried everything I could
> > think of, but invariably as soon as I start invoking the quickCheck
> > function on my property it fails with
> >
> > <interactive>:1:0:
> >     No instances for (Arbitrary (OrdSeq Int), Show (OrdSeq Int))
> >       arising from use of `quickCheck' at <interactive>:1:0-20
> >     Probable fix:
> >       add an instance declaration for (Arbitrary (OrdSeq Int), Show
> > (OrdSeq Int))
> >     In the definition of `it': it = quickCheck prop_DeleteInsert
> >
> > The property is quite a simple one:
> >
> > prop_DeleteInsert x xs = delete x (insert x xs) == xs
> >   where types = xs :: OrdSeq Int
> >
> > My code compiles just fine with ghc.
>
> It compiles (and runs) fine as a function on OrdSeqs, with type
>
>     prop_DeleteInsert :: Int -> OrdSeq Int -> Bool
>
> But as an argument to quickCheck, it needs be in the class Testable,
> which boils down to the requirement that Int and "OrdSeq Int" are
> both in the class Arbitrary.  So you need to write something like
>
>     instance Arbitrary a => Arbitrary (OrdSeq a) where ...
>
> This is confusing because QuickCheck is clever with type classes to
> check "ordinary" boolean functions.  But in fact, your property is
> equivalent to
>
>     prop_DeleteInsert = forall arbitrary $ \x ->
>                           forall arbitrary $ \xs ->
>                             delete x (insert x xs) == (xs :: OrdSeq
> Int)
>
> And QuickCheck doesn't know how to create arbitrary OrdSeqs unless
> you tell it.

Yes, now as you say it it seems quite obvious. I was mislead by the 
other error message, which I took to be the more important one, because 
it appeared first. Bad habits...

> > I downloaded the haskell script 'quickCheck' and modified the line
> > where ghci is executed like this:
> >
> > system ("/usr/bin/ghci -cpp -package QuickCheck "++options opts'++"
> > <hugsin")
> >
> > (I am using ghc 6.4.1, btw). Running 'quickCheck
> > Data/FingerSearchTreeUtil.hs' gives me:
> >
> > <interactive>:1:0:
> >     Failed to load interface for `QuickCheck':
> >         Could not find module `QuickCheck':
> >           it is a member of package util-1.0, which is hidden
> >
> > Strange. Replacing 'ChickCheck' with 'util' gives me the above
> > error message about "No instance for (Show (IO ()))..."
>
> In package QuickCheck, the module is called "Test.QuickCheck" (so
> "import Test.QuickCheck"), whereas in package util, it is called just
> "QuickCheck" (but this is deprecated).

Thanks a lot for this explanation! Taking a look at the script, I see 
"QuickCheck.quickCheck" being written to the program that ghci gets as 
input. Changing it to "Test.QuickCheck.quickCheck" indeed fixes the 
problem.

Thanks everyone for helping,
Ben