CODEWITHSUNDEEP
haskelltypes
harleengulati03
harleengulati03

Reputation: 25

Defining types in Haskell in terms of other types and typeclasses struggling

Can someone please explain to me how we can define a new type in terms of itself in Haskell. Below is the snippet of code I am struggling to understand. I do not understand how we can define a new type in terms of itself. We are saying Card is a new type with constructors Card Rank Suit. What does this even mean?

Any help would be greatly appreciated.

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

data Rank = Numeric Int | Jack | Queen | King | Ace
              deriving Show

data Card = Card Rank Suit
              deriving Show

Upvotes: 0

Views: 82

Answers (1)

leftaroundabout
leftaroundabout

Reputation: 120751

We are saying Card is a new type with constructors Card Rank Suit. What does this even mean?

Well, first of all it's wrong. Card is a new type with the single constructor Card; the latter takes two arguments, which are of type Rank and Suit.

The confusing thing here is that you're really dealing with two different things that are both called Card. Let's disambiguate:

data CardT = CardC Rank Suit
              deriving Show

That expresses the same definition, but now it's clear that the type and constructor aren't the same thing. CardT lives in the type-level language, CardC lives in the value-level language.

> :t CardC
CardC :: Rank -> Suit -> CardT

Upvotes: 2

Related Questions