scala3-Matchable-dependently-typed-method.scala

// Adapted from Programming Scala, Third Edition code examples.
// https://github.com/deanwampler/programming-scala-book-code-examples
// src/script/scala/progscala3/typesystem/matchtypes/DepTypedMethods.scala

type ElemR[X] = X match         // "R" for "recursive"
  case String => Char
  case Array[t] => ElemR[t] 
  case Iterable[t] => ElemR[t]
  case Option[t] => ElemR[t]
  case AnyVal => X

import compiletime.asMatchable                                       

def first[X](x: X): ElemR[X] = x.asMatchable match                   
  case s: String      => s.charAt(0)
  case a: Array[t]    => first(a(0))
  case i: Iterable[t] => first(i.head)
  case o: Option[t]   => first(o.get)
  case x: AnyVal      => x

// Define some types
case class C(name: String)
object O

first("one")                //  val res0: Char = o
first(Array(2.2, 3.3))      //  val res1: Double = 2.2
first(Seq("4", "five"))     //  val res2: Char = 4
first(6)                    //  val res3: Int = 6
first(true)                 //  val res4: Boolean = true
first(O)                    //  val res5: ElemR[O.type] = O$@4853f592
first(C("Dean"))            //  val res6: ElemR[C] = C(Dean)