Tree parameterized over the root, with an arbitrary type in the child-nodes.

galet.hs

{-# LANGUAGE ExistentialQuantification #-}
import Data.List (unlines)
data HasEq = forall a. (Eq a, Show a) => HasEq a
data AnyIntegral = forall a. Integral a => Integral a

data AnyTree x = forall a b. (Show a, Show b) => Tree x (AnyTree a) (AnyTree b) | Nil

instance Show a => Show (AnyTree a) where
    show (Tree x a b) = show (x,(a,b))
    show Nil = "Nil"

instance Show HasEq where
    show (HasEq x) = "HasEq " ++ show x

--peal :: HasEq -> Int
peal (Integral x) = fromIntegral x