08-making-our-own-types-and-typeclasses.hs

import Data.List
{-
 - We are going to create some types for a deck of cards
 - The cards need to have an ordering, based on the standard ranking http://en.wikipedia.org/wiki/Standard_52-card_deck#Rank_and_color
 - We are assuming Aces are high.
 - Therefore, the following statements should be true:
 -    (Card Ace Spades) > (Card King Spades)
 -    (Card Two Clubs) < (Card Three Clubs)
 -
 - We are going to provide our own implementation of the Show typeclass for the Card type.
 - When displaying the Card instance in GHCI, or calling show (Card digit suit), the String which should be displayed is "The <Digit> of <Suit>"
 -
 - Uncomment the following declarations to complete the implementation, and provide an implementation for instance Show Card
 -}

data Suit = Clubs | Diamonds | Hearts | Spades deriving (Show, Eq, Ord)
data Digit = Two|Three|Four|Five|Six|Seven|Eight|Nine|Ten|Jack|Queen|King|Ace deriving (Show, Eq, Ord)
data Card = Card {digit :: Digit, suit :: Suit} deriving (Eq, Ord)
instance Show Card where
    show (Card {digit = d, suit = s}) = "The " ++ show d ++ " of " ++ show s

-- We should be able to provide a function which returns the higher ranked card:
betterCard :: Card -> Card -> Card
betterCard x y = if x >= y then x else y

-- Here is a new Typeclass, which represents some kind of playing hand in a game.
-- It returns True for a "winning hand", depending on the rules for the type of class we are playing with
class Hand a where
    play :: [a] -> Bool

-- Implement Hand for Card, where play returns true if the list contains the Ace of Spades
instance Hand Card where
    play c = elem (Card Ace Spades) c

-- Create a new Coin type
data Coin = Heads | Tails deriving (Show, Eq) 

-- Implement Hand for Coin, where play returns true if there are ten heads in a row in the list
instance Hand Coin where
    play c = elem True [elem Heads cgroup && length cgroup >= 10 | cgroup <- group c]  

-- -- Have a play with implementing Hand for some other types, for instance Int and Bool


instance Hand Int where
    play i = sum i == product i

instance Hand Bool where
    play b = elem True b