Exercise – Making a Card Deck Type

Let's now try to implement a data type that will represent a deck of cards. First, we can think about what type would be fitting for a card deck – a list of cards would be a good representation. But then what type is fitting for a single card? Each card should have a rank and a suit so we can make another type for cards that has the type of a tuple (Rank, Suit). Let's start at the lowest level, the Rank and Suit type. Remember that we already defined the Suit type, but we will now also derive the Show class for it:

data Suit = Hearts
  | Diamonds
  | Spades
  | Clubs
    deriving (Show)

which leaves us with the task of defining Rank for which we can also use nullary constructors and also derive some built-in classes:

data Rank = Deuce
  | Three
  | Four
  | Five
  | Six
  | Seven
  | Nine
  | Ten
  | Jack
  | Queen
  | King
  | Ace
    deriving (Show, Eq, Ord, Enum)

We can then already use methods of those classes on the Rank type:

ghci> Deuce < Three
True

ghci> Deuce <= Three
True

ghci> Deuce == Three
False

ghci> Deuce > Three
False

ghci> [Deuce .. Five]
[Deuce, Three, Four, Five]

We have the Rank and Suit types now, and we can simply define the type of Card as a type synonym for a tuple of (Rank, Suit):

type Card = (Rank, Suit)

Similarly, we can define a deck of cards as a type synonym for a list of Card types:

type Deck = [Card]

We have all the required types for actually building a deck now, so let's make a function for that purpose. We will use list comprehension and take advantage of the fact that Rank supports enumeration:

buildDeck :: Deck
buildDeck = [(rank, suit) | rank <- [Deuce .. Ace], suit <- suitList]
  where
    suitList = [Hearts, Diamonds, Spades, Clubs]

And we can now build a deck of cards using that function:

ghci> deck = buildDeck
ghci> show deck
"[(Deuce, Hearts),(Deuce, Diamonds),(Deuce, Spades),(Deuce, Clubs),
(Three, Hearts),(Three, Diamonds),(Three, Spades),(Three, Clubs),(Four, 
Hearts),(Four, Diamonds),(Four, Spades),(Four, Clubs),(Five, Hearts),
(Five, Diamonds),(Five, Spades),(Five, Clubs),(Six, Hearts),(Six, 
Diamonds),(Six, Spades),(Six, Clubs),(Seven, Hearts),(Seven, Diamonds),
(Seven, Spades),(Seven, Clubs),(Nine, Hearts),(Nine, Diamonds),(Nine, 
Spades),(Nine, Clubs),(Ten, Hearts),(Ten, Diamonds),(Ten, Spades),(Ten, 
Clubs),(Jack, Hearts),(Jack, Diamonds),(Jack, Spades),(Jack, Clubs),
(Queen, Hearts),(Queen, Diamonds),(Queen, Spades),(Queen, Clubs),(King, 
Hearts),(King, Diamonds),(King, Spades),(King, Clubs),(Ace, Hearts),(Ace,
Diamonds),(Ace, Spades),(Ace, Clubs)]"