How can I typecheck custom datatypes in Haskell?-CodePudding

I'm very new to haskell, so sorry in advance.

I'm writing a program thats has a custom type "Rank". (The rank of a blackjack-card)

data Rank = Int | Jack | Queen | King | Ace

data Suit = Club | Diamond | Heart | Spade

data Card = Card Rank Suit


getCardValue :: Card -> Int
getCardValue (Card val _) = valueOfRank val

valueOfRank :: Rank -> Int
valueOfRank (Int i) = i --Doesnt work
valueOfRank Jack = 10
valueOfRank Queen = 10
valueOfRank King = 10
valueOfRank Ace = 11

Now I want to receive the respective value of a card with a function, my problem is, that I dont know how to typecheck if the value is of the type Int.

CodePudding user response：

data Rank = Int | Jack | Queen | King | Ace

The above Int is a data constructor name, and is completely unrelated to the Int type. Indeed, Jack is a similar data constructor but there's no Jack type around.

If you want a Rank value actually containing a value of type Int, you need to use something like

data Rank = I Int | Jack | Queen | King | Ace

Here I is the constructor name (you can rename it as you wish), and Int now refers to the type. You can then use Rank as:

valueOfRank :: Rank -> Int
valueOfRank (I i) = i
valueOfRank Jack = 10
valueOfRank Queen = 10
valueOfRank King = 10
valueOfRank Ace = 11

Note that nothing prevents the programmer to abuse the I constructor and create invalid card values like I 123, I 11, and I 1.