Can you formulate a case expression in a function that will accept the datatype cards and will return a boolean?

Question

datatype cards = king of int * int
               | queen of string
               | jack of cards
               | ace of cards * cards
               | joker of int * cards

Answer 1

Sure.

fun hasKing cards =
    case cards of
         king (i, j)          => true
       | queen s              => false
       | jack cards1          => hasKing cards1
       | ace (cards1, cards2) => hasKing cards1 orelse hasKing cards2
       | joker (i, cards2)    => hasKing cards2

However, I would name my value constructors in uppercase to differentiate them from functions:

datatype cards = King of int * int
               | Queen of string
               | Jack of cards
               | Ace of cards * cards
               | Joker of int * cards