Data.Random.Extras Usage - list

Question

I'm trying to use a shuffle function from Data.Random.Extras package on the list of cards I have created. My code:
module Cards where
import Data.Random.Extras
data Suit = Clubs
| Diamonds
| Hearts
| Spades
deriving (Eq,Enum,Ord,Show,Bounded)
data Value = Two
| Three
| Four
| Five
| Six
| Seven
| Eight
| Nine
| Ten
| Jack
| Queen
| King
| Ace
deriving (Eq,Enum,Ord,Show,Bounded)
data Card = Card Value Suit
deriving (Eq,Ord,Show)
type Deck = [Card]
-- Generate deck of cards
generateDeck :: Deck
generateDeck = [Card val suit | suit <- [Clubs .. Spades], val <- [Two .. Ace]]
-- Print deck of cards
printDeck :: Deck -> IO ()
printDeck deck = putStr (formatDeck deck)
where
formatDeck [] = []
formatDeck (x:xs) = (show x) ++ "\n" ++ formatDeck xs
The problem is that when I'm trying to execute shuffle $ generateDeck on GHCi prompt I'm getting:
No instance for (Show (Data.RVar.RVar [Card]))
arising from use of 'print'
Possible fix:
Add an instance declaration for (Show (Data.RVar.RVar [Card]))
In a stmt of an interactive GHCi command: print it
I spent hours searching and trying to solve/understand this without any success. I would really appreciate your help.
Thank you.

Answer 1

That is because the type of shuffle is [a] -> RVar [a]. So you don't get back an [a] which you could use directly but some mysterious RVar. RVar is a bit like IO in that an RVar a is not a value of type a but more of a mean of getting a value of type a. In order to actually get a value of type a you have to use runRVar :: RandomSource m s => RVar a -> s -> m a. Looking at the documentation of RandomSource, there are multiple instances for it. One example would be MonadRandom m => RandomSource m StdRandom. So we can use runRVar like this (because IO is an instance of MonadRandom):
> import Data.Random (runRVar, StdRandom(..))
> import Data.Random.Extras (shuffle)
> runRVar (shuffle [1..10]) StdRandom :: IO [Int]
[3,10,8,5,6,7,4,2,9,1]
Or another instance of MonadRandom, that doesn't involve IO would be State StdGen a:
> -- Above imports and
> import Control.Monad.State (State,evalState)
> import System.Random (StdGen,mkStdGen)
> shuffledState = runRVar (shuffle [1..10]) StdRandom :: State StdGen [Int]
> evalState shuffledState $ mkStdGen 0 -- 0 is the seed
[6,8,7,5,10,9,2,3,1,4]