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

Question

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://www.haskell.org/ghc/ :? 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

Answer 1

Don't define a new Arbitrary type class, import Test.QuickCheck instead. It defines most of these instances for you. Also be careful about the version of quickcheck, RWH assumes version 1.
The resulting full implementation will be:
-- file: ch11/Arbitrary.hs
import Test.QuickCheck
import Prettify2
import Control.Monad( liftM, liftM2 )
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 Doc where
arbitrary =
oneof [ return Empty
, liftM Char arbitrary
, liftM Text arbitrary
, return Line
, liftM2 Concat arbitrary arbitrary
, liftM2 Union arbitrary arbitrary ]