QuickCheck prop to tasty prop

Question

Im trying to go from from _prop example to write the same thing in Tasty. (example from http://primitive-automaton.logdown.com/posts/142511/tdd-with-quickcheck)

game9_prop :: Game9 -> Bool
game9_prop = (9==) . length . unGame . unGame9

This is what I'm trying in tasty:

qcTestGengame9 :: TestTree
qcTestGengame9 = QC.testProperty "Better gen rate" $ 
   \ (g :: Game9) -> length . unGame . unGame9 g == 9 --ERROR LINE

This conversion gives me the following error:

test/Tasty.hs:53:11:
Illegal type signature: `Game9'
Perhaps you intended to use -XScopedTypeVariables
In a pattern type-signature

This is the type Game9:

-- To make generation rate better
newtype Game9 = Game9 { unGame9 :: Game }
   deriving (Eq, Show)

instance Arbitrary Game9 where
   arbitrary = (Game9 . Game) `liftM` replicateM 9 arbitrary

Answer 1

To fix the immediate error, remove the type annotation, i.e., use

qcTestGengame9 :: TestTree
qcTestGengame9 = QC.testProperty "Better gen rate" $ 
   \ g -> (length . unGame . unGame9) g == 9

The expression unGame9 g already ensures g :: Game9.

But actually it's simpler: with game9_prop as defined, you can just use

qcTestGengame9 :: TestTree
qcTestGengame9 = QC.testProperty "Better gen rate" game9_prop