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Case Expressions

Syntax​

case expression of
                pattern -> result
                pattern -> result
                pattern -> result

case expression and pattern matching​

pattern matching in function definitions is syntactic sugar for case expressions. Rewite the previous pattern matching example code:

lucky' :: Int -> String
lucky' xs = case xs of
                    7 -> "LUCKY NUMBER SEVEN!"
                    x -> "Sorry, you're out of luck, pal!"

  • case expressions are expressions, can be used pretty much anywhere.

  • pattern matching on function parameters can only be done when defining functions.

    describeList :: forall a.Array String -> String
    describeList xs = "The array is " <> case xs of
      [] -> "empty."
      [x] -> "a singleton array."
      xs -> "a longer array."

    load and run in repl:

    > describeList []
    "The list is empty."

    > describeList ["a"]
    "The list is a singleton list."

    > describeList ["a", "b", "c"]
    "The list is a longer list."