needs a better name

Arbiter.hs

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

module Main where

import Test.QuickCheck

class Arbiter f r where
  arbiter :: f -> Gen r

instance Arbiter r r where
  arbiter = return

instance (Arbitrary a, Arbiter f r) => Arbiter (a -> f) r where
  arbiter f = do
    a <- arbitrary
    arbiter (f a)

data X = X Int Int Int Int Int

instance Show X where
  show _ = "X"

instance Arbitrary X where
  arbitrary = arbiter X

main = quickCheck $ \(X _ _ _ _ _) -> True