Hello, I was playing with the following tree type (attached below) which can be seen as the reification of an applicative. I wondered if I could define a QuickCheck Arbitrary instance for it. The only way I got it to type check however, was to give 'arg' a monomorphic type (for example: 'Gen (Tree ())'). If I left it polymorphic I got a "Ambiguous type variable in constraints" error. This is understandable because if the subtrees have polymorphic types they can be of any type: Tree Int, Tree Float, Tree String, etc. The system then doesn't know which Arbitrary to choose. My question is, is it possible to keep 'arg' polymorphic (i.e. 'Gen (Tree b)') and let the system somehow choose an arbitrary Arbitrary? I guess not, however I like to be proven wrong. regards, Bas ---------------------------------------------------- {-# LANGUAGE ExistentialQuantification #-} module Tree where import Control.Monad import Test.QuickCheck -- I'm using QuickCheck-2.1.0.1 data Tree a = Val a | forall b. Tree (b -> a) :*: Tree b instance Arbitrary a => Arbitrary (Tree a) where arbitrary = sized sizedTree sizedTree :: (Arbitrary a) => Int -> Gen (Tree a) sizedTree n | n <= 0 = liftM Val arbitrary | otherwise = liftM2 (:*:) func arg where m = n `div` 2 func = sizedTree m arg :: Gen (Tree ()) -- how to make this: -- Gen (Tree b) ??? arg = sizedTree m ----------------------------------------------------