Haskell: Value of a Card, function-CodePudding

I'm still doing the exercise from the book and the one exercise says that I should create a function: value :: Hand -> Int, which returns the value of the hand.

My code so far looks like this:

data Card = Card Rank Suit 
            deriving (Show)

data Suit = Spade | Heart | Diamond | Club
            deriving (Show, Ord, Eq, Enum)

data Rank = Ace | King | Queen | Jack | Ten | Nine | Eight | Seven
            deriving (Show, Eq, Ord, Enum)

data Hand = PairOf Rank | ThreeOf Rank | Flush Suit
            deriving (Show, Eq)

-- This function implements a value function which returns the value of the hand.
-- Pair (2 cards of the same rank) = 1
-- ThreeOf (3 cards of the same rank) = 2
-- Flush (3 cards of the same suit) = 3
-- Nothing = 0

value :: Hand -> Int
value (PairOf r1) = 1
value (Drilling r1) = 2
value (Flush s1) = 3
otherwise = 0

I now have another problem, because I don't know how to describe "Nothing". Nothing is described here as a possible hand combination in which Pair, ThreeOf and Flush do not occur. Would otherwise = 0 work or doesn't that make much sense?

Thanks again for the suggestions, I corrected them! Thanks also in advance for explanations and help.

CodePudding user response：

otherwise won't work here. What you want is an irrefutable pattern that will match anything, and further since you don't care what gets matched, you specifically want the pattern _:

value :: Hand -> Int
value (PairOf r1) = 1
value (ThreeOf r1) = 2
value (Flush s1) = 3
value _ = 0

Since you don't care about what kind of pair, etc you get, you can also use _ in the other cases:

value :: Hand -> Int
value (PairOf _) = 1
value (ThreeOf _) = 2
value (Flush _) = 3
value _ = 0

If you wanted to match Nothing (or whatever name you come up with that doesn't conflict with the Maybe constructor) specifically, it's just

value Nothing = 0