Arbitrary String generator in Haskell (Test.QuickCheck.Gen)

I am struggling on Real World Haskell Chapter 11 quickCheck generator implementation for a an algebraic data type.

Following the book implementation (which was published in 2008), I came up with the following:

-- file: ch11/Prettify2.hs
module Prettify2(
    Doc(..)
) where

data Doc = Empty
         | Char Char
         | Text String
         | Line
         | Concat Doc Doc
         | Union Doc Doc
         deriving (Show, Eq)

And my Arbitrary implementation:

-- file: ch11/Arbitrary.hs

import System.Random
import Test.QuickCheck.Gen
import qualified Test.QuickCheck.Arbitrary


class Arbitrary a where
    arbitrary :: Gen a
    -- elements' :: [a] => Gen a {- Expected a constraint, but ‘[a]’ has kind ‘*’ -}
    -- choose' :: Random a => (a, a) -> Gen a
    -- oneof' :: [Gen a] -> a

data Ternary = Yes
             | No
             | Unknown
             deriving(Eq, Show)

instance Arbitrary Ternary where
    arbitrary = do
        n <- choose (0, 2) :: Gen Int
        return $ case n of
                      0 -> Yes
                      1 -> No
                      _ -> Unknown

instance (Arbitrary a, Arbitrary b) => Arbitrary (a, b) where
  arbitrary = do
      x <- arbitrary
      y <- arbitrary
      return (x, y)

instance Arbitrary Char where
    arbitrary = elements (['A'..'Z'] ++ ['a' .. 'z'] ++ " ~!@#$%^&*()")

I tried the two following implementation with no success:

import Prettify2
import Control.Monad( liftM, liftM2 )

instance Arbitrary Doc where
    arbitrary = do
        n <- choose (1,6) :: Gen Int
        case n of
            1 -> return Empty
            2 -> do x <- arbitrary
                    return (Char x)
            3 -> do x <- arbitrary
                    return (Text x)
            4 -> return Line
            5 -> do x <- arbitrary
                    y <- arbitrary
                    return (Concat x y)
            6 -> do x <- arbitrary
                    y <- arbitrary
                    return (Union x y)

instance Arbitrary Doc where
    arbitrary =
        oneof [ return Empty
              , liftM Char arbitrary
              , liftM Text arbitrary
              , return Line
              , liftM2 Concat arbitrary arbitrary
              , liftM2 Union arbitrary arbitrary ]

But it doesn't compile since No instance for (Arbitrary String)

I tried then to implement the instance for Arbitrary String in the following ways:

• import qualified Test.QuickCheck.Arbitrary but it does not implement Arbitrary String neither

• installing Test.RandomStrings hackage link

  instance Arbitrary String where arbitrary = do n <- choose (8, 16) :: Gen Int return $ randomWord randomASCII n :: Gen String

With the following backtrace:

$ ghci
GHCi, version 7.10.3: http://ift.tt/RDyt1t  :? for help
Prelude> :l Arbitrary.hs
[1 of 2] Compiling Prettify2        ( Prettify2.hs, interpreted )
[2 of 2] Compiling Main             ( Arbitrary.hs, interpreted )

Arbitrary.hs:76:9:
    The last statement in a 'do' block must be an expression
      return <- randomWord randomASCII n :: Gen String
Failed, modules loaded: Prettify2

Would you have any good suggestion about how to implement this particular generator and - more in general - how to proceed in these cases?

Thank you in advance