FCards - Defining Cards

Records

A record is a type of thing that has multiple parts to its value, like a name, or id, or a number.

Each of these parts has a label and a type, which itself can even be another record type. Traditionally, types of things (like a record definition) start with a capital letter, and instances of these things (e.g a variable) start with a small letter.

Parts of a type are separated with a semicolon (;) or are put on separate lines at the next level of indentation

So, we can define a playing Card as having a couple of parts - the suit and the number value

type Card = {
  suit: string
  value: int
}

let myCard = { suit = "Clubs"; value=3 }

Discriminated Union

A discriminated union (DU for short) is a type of thing that can be either of its parts, but only one at a time.

For instance a DU for the suits of a deck of cards would be…

type Suit = 
  | Hearts
  | Diamonds
  | Clubs 
  | Spades

type Card = {
  suit: Suit
  value: int
}

let myCard = { suit = Clubs; value=3 }

A more complex form of a DU can specify a type for each of the parts. The types of the parts don’t have to be the same.

type CardNumber =
  | Two 
  | Three
  | Four
  | Five
  | Six
  | Seven
  | Eight
  | Nine
  | Ten
  | Jack
  | Queen
  | King
  | Ace

type Card = 
  | Hearts of CardNumber
  | Diamonds of CardNumber
  | Clubs of CardNumber
  | Spades of CardNumber
  | Joker



let myCard = Hearts Three

TIP: The labels for a DU have to start with a capital letter, so we can’t just use the number for the label directly.

Code so far

type CardNumber =
  | Two 
  | Three
  | Four
  | Five
  | Six
  | Seven
  | Eight
  | Nine
  | Ten
  | Jack
  | Queen
  | King
  | Ace

type Card = 
  | Hearts of CardNumber
  | Diamonds of CardNumber
  | Clubs of CardNumber
  | Spades of CardNumber
  | Joker