# [Haskell-cafe] Dealing poker hands, redux

Paolino paolo.veronelli at gmail.com
Sat Aug 25 15:09:05 CEST 2012

```hello matthew,

I commented your gist on github adding the missing pieces with minor fixes.

Anyway, I suggest giving up RVar's which are overkill for your task. You
could implement your shuffle as an exercise, or use random-shuffle package
as I do in the code below.

Also I'd avoid State monad for such a simple task of remembering the
remaining deck after each deal.  You should switch to using a monad after
you are comfortable with pure code.

import Control.Arrow
import System.Random.Shuffle

-- |Cards.
data Suit = Hearts | Diamonds | Clubs | Spades deriving (Enum, Bounded,
Show)

data Rank = Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine |
Ten | Jack | Queen | King deriving (Enum, Bounded, Show)

data Card = Flat Rank Suit | Jolly deriving Show

type Deck = [Card]
type Hand = [Card]

deck :: Deck
deck = [ Flat s r | s <- [minBound .. maxBound] , r <- [minBound ..
maxBound]]++ [Jolly,Jolly]

-- deal n cards m times from the given deck and return the rest along the
hands
dealHands :: Int -> Int -> Deck -> ([Hand],Deck)
dealHands n m  = (map (take n) *** head) . splitAt m . iterate (drop n)

-- same as dealHands, but shuffle the deck before
randomDealHands :: (Functor m, MonadRandom m) => Int -> Int -> Deck -> m
([Hand],Deck)
randomDealHands n m xs = dealHands n m `fmap` shuffleM xs

test :: IO ([Hand],Deck)
test = randomDealHands 5 4 \$ deck ++ deck

You can run randomDealHands in IO (ghci prompt) as IO is both a Functor and

regards
paolino
2012/8/25 Matthew <wonderzombie at gmail.com>

> So I've finally got some code which shuffles a deck of cards and deals
> out an arbitrary number of hands.
>
> https://gist.github.com/19916435df2b116e0edc
>
> type DealerState = State [Card] [[Card]]
>
> deck :: [Card]
> deck = [ (s, r) | s <- suits, r <- ranks ]
>
> shuffleDeck :: Int -> RVar [Card]
> shuffleDeck n = shuffle \$ concat \$ replicate n deck
>
> deal :: Int -> ([[Card]] -> DealerState)
> deal n = \xs -> state \$ \s -> (xs ++ [take n s], drop n s)
>
> -- |Deal a number of hands a number of cards each.
> dealHands :: Int -> Int -> ([[Card]] -> DealerState)
> dealHands hs cs = foldr1 (<=<) \$ replicate hs (deal cs)
>
> First of all, I have no idea if this is any good. The way I end up
> calling dealHands and getting a "real" result is `runState (dealHands
> 3 7 []) deck`. And I see that I've got nested lambdas all in `deal`.
> But hey it took me forever to figure it out and it "works."
>
> I'm using `shuffle` from Data.Random.Extras, which results in an RVar,
> and thus the beginning of my perplexity. I'm happy to end up with RVar
> inside a State monad, but I'm not sure where to start. To be honest,
> I'm only barely understanding what I'm doing with the State monad as
> it is. :)
>
> Happy for any help whatsoever!
>
> - Matthew
>
> _______________________________________________