On Friday 21 October 2005 20:14, Udo Stenzel wrote:
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
No instances for (Arbitrary (OrdSeq Int), Show (OrdSeq Int))
You need to define an `instance Arbitrary (OrdSeq Int)' for QuickCheck to be able to generate test data. `instance Show (OrdSeq Int)' is needed so the offending example can be reported to you in case some test fails. You should believe your compiler when it tells you
add an instance declaration for (Arbitrary (OrdSeq Int), Show (OrdSeq Int))
;-)
"No instance for (Show (IO ()))..."
I think, this is an artefact of unresolved overloading or missing instances. Somehow GHCi attaches an unnecessary implicit `print'. When I played with QuickCheck, this error went away as soon as the rest of the expression type checked.
Many thanks! I wish I had remembered that I had this kind of problem before: ghc reports two errors, of which the /second/ one is the 'real' problem and the first one is just a by-product. This practically never happens with C compilers which I (must, sadly) use every day and thus I always tend to disregard all error messages but the first. And now as I think of it, it would indeed be quite hard for QuickCheck to guess hwo to generate my OrdSeqs. Ben