match Expressions, Part 2 (Scala 3 Video)

This lesson looks at more advanced uses of Scala match expressions.

Matching multiple patterns on one line

Match multiple patterns on one line (in one case):

def isTrue(a: Matchable): Boolean = a match     // VERSION 2
    case 0 | "" | "0" | false => false
    case _     => true

// another example of multiple patterns
i match
    case 1 | 3 | 5 | 7 | 9 => "odd"
    case 2 | 4 | 6 | 8 | 10 => "even"
    case _ => "other"

Giving the default case a variable name

How to give the default catch-all case a name:

i match
    case 1 | 3 | 5 | 7 | 9 => "odd"
    case 2 | 4 | 6 | 8 | 10 => "even"
    case default => s"You gave me a $default"

Using case classes in match expressions

case classes and match expressions:

// A function that takes `SentientBeing`
// How to extract info from `case` classes
// The cases are exhaustive because of the sealed traits

sealed trait SentientBeing
sealed trait Animal extends SentientBeing
case class Dog(name: String) extends Animal
case class Person(name: String, age: Int) extends SentientBeing
// later in the code ...
def getInfo(sb: SentientBeing): String = sb match
    case Person(name, age) => 
        s"Person, name = $name, age = $age"
    case Dog(name) => 
        s"Dog, name = $name"

Using a match expression as the body of a function

Using a match expression as the body of a function, in this case a recursive function:

// recursive (but not tail-recursive)
def sum(list: List[Int]): Int = list match
    case Nil => 0
    case head :: tail => head + sum(tail)

// same example, but with added debug comments
def sum(xs: List[Int]): Int = xs match
    case Nil =>
        println("Nil")
        0
    case head :: tail =>
        println(s"head = $head, tail = $tail")
        head + sum(tail)

Using match with Option/Some/None

Using with Option/Some/None:

val allow: Boolean = ageAsOption match
    case Some(i) => i >= 21
    case None    => false

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