Discussion on: When to use a Discriminated Union vs Record Type in F#

This is a good explanation of the two types.

Both types are very useful on there own, but things get interesting when you combine them. Also, there are choices for how to combine them.

For example, in a card game of sorts is there a record type Card that has a discriminated union CardType or does the union contain the records?

See the following:

type CardType
| Attack of int
| Defense of int

type Card = {
  cost: int;
  type: CardType;
}

or

type AttackCard =
{
  strength: int;
  cost: int;
}

type DefenseCard =
{
  mitigation: int;
  cost: int;
}

type Card
| Attack of AttackCard
| Defense of DefenseCard

I think it depends on how the cards are going to be used. If the second pattern is followed then every interaction with a card will first have to handle the card type. If you need the cost before the type something like the first pattern should work.