Re: [Haskell-cafe] Help using QuickCheck

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'++"

(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