A Scala translation of James Ward's ContextReceivers example

adhocpoly.scala

// A literal Scala translation of James Ward's Context receivers example
// https://github.com/jamesward/oop-evolution/blob/context-receivers/gradleable/src/main/kotlin/adhocpoly/ContextReceivers.kt
// I had to rename `sum` to `summ` since `sum` already exists as a method on lists in Scala.

package adhocpoly

trait Summable[T]:
  def tplus(t1: T, t2: T): T
object Summable:
  def tplus[T](t1: T, t2: T)(using s: Summable[T]) = s.tplus(t1, t2)

extension [T: Summable](xs: Iterable[T])
  def summ: T = xs.reduce(Summable.tplus)

@main def test =
  given Summable[Char] with
    def tplus(t1: Char, t2: Char): Char =
      (t1 + t2 - ('a' - 1)).toChar

  given Summable[String] with
    def tplus(t1: String, t2: String): String =
      t1.lazyZip(t2).map(Summable.tplus)

  println(List('a', 'b').summ)
  println(List("ah", "he").summ)

This was as close as possible to James Ward's version in https://github.com/jamesward/oop-evolution/blob/context-receivers/gradleable/src/main/kotlin/adhocpoly/ContextReceivers.kt.

A more idiomatic Scala version would use an infix method for the combine operation, like this:

package adhocpoly

trait Summable[T]:
  def tplus(t1: T, t2: T): T
  extension (t1: T) infix def combine (t2: T): T = tplus(t1, t2)

extension [T: Summable](xs: Iterable[T])
  def summ: T = xs.reduce(_ combine _)

@main def test =
  given Summable[Char] with
    def tplus(t1: Char, t2: Char): Char =
      (t1 + t2 - ('a' - 1)).toChar

  given Summable[String] with
    def tplus(t1: String, t2: String): String =
      t1.lazyZip(t2).map(_ combine _)

  println(List('a', 'b').summ)
  println(List("ah", "he").summ)