Variadic function type in haskell

variadic.hs

class Foo t where
  returnLast :: Int -> t

class Integral i => IntsBro i where
  toInt :: i -> Int

instance IntsBro Integer where
-- toInt :: Integer -> Int
  toInt i = fromIntegral i :: Int

instance Foo Int where
-- returnLast :: Int -> Int
  returnLast = id

instance (IntsBro a, Foo b) => Foo (a -> b) where
-- returnLast :: Int -> a -> b
  returnLast _ = returnLast . toInt . toInteger

returnLast 1 2 3 4 :: Int --will return 4
returnLast 1 2 :: Int --will return 2, etc