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08TypesAndTypeclasses.hs
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08TypesAndTypeclasses.hs
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{-
- 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 = Spades | Clubs | Diamonds | Hearts deriving (Eq, Ord, Show)
data Digit = Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten | Jack | Queen | King | Ace deriving (Eq, Ord, Show)
data Card = Card Digit Suit deriving (Eq, Ord)
instance Show Card where
show (Card dig suit) = (show dig) ++ " of " ++ (show suit)
-- We should be able to provide a function which returns the higher ranked card:
betterCard :: Card -> Card -> Card
betterCard x y
| x > y = x
| y > x = 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 = (Card Ace Spades) `elem` 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 = play' c 10
play' :: [Coin] -> Int -> Bool
play' [] _ = False
play' [x] n
| n == 1 = if x == Heads then True else False
| otherwise = False
play' (x:xs) 1
| x == Heads = True
| otherwise = False
play' (x:xs) n
| x == Heads = play' xs (n-1)
| otherwise = play' xs 10
-- Have a play with implementing Hand for some other types, for instance Int and Bool