Rose.hs

instance Arbitrary a => Arbitrary (Rose a) where
  arbitrary =
    let
      numChildren :: Gen Int
      numChildren = (`mod` 6) . abs <$> arbitrary

      makeRose :: Arbitrary a => Int -> Gen (Rose a)
      makeRose n = rose $ n `div` 2

      branch :: Arbitrary a => Int -> Gen [Rose a]
      branch n =
        range >>= mapM (const (makeRose n))
        where
          range = flip take [0..] <$> numChildren

      rose :: Arbitrary a => Int -> Gen (Rose a)
      rose 0 = MkRose <$> arbitrary <*> return []
      rose n = MkRose <$> arbitrary <*> branch n
    in
      sized rose