Case-of Statements

There is another type of conditional statement in Haskell - case-of. It uses pattern matching to determine the expression to be evaluated. If you are familiar with switch statements from imperative programming, this is their equivalent in Haskell. The syntax is:

case <EXPRESSION> of
  <PATTERN1> -> <EXPRESSION1>
  <PATTERN2> -> <EXPRESSION2>
  ...
  <PATTERNx> -> <EXPRESSIONx>
  _          -> <DEFAULT_EXPRESSION>

The _ is a wildcard character. It is a useful tool for when we do not really care about what the value of the expression might be. In this case, whatever that value is – we know what we want to do if none of our previous patterns matches and assign it the default expression. For example, we could define a function that returns the colour of a playing card based on its suit:

cardColour :: String -> String
cardColour suit =
  case suit of
    "hearts" -> "red"
    "diamonds" -> "red"
    "spades" -> "black"
    "clubs" -> "black"
    _ -> "I am not familiar with this card suit."
    
ghci> cardColour "diamonds"
"red"
ghci> cardColour "ace"
"I am not familiar with this card suit."

That is, for the four valid suits we return their respective colours. Anything else is covered by the wildcard case and no matter what the value is, we always choose the same course of action.