New DSL ideas for Scala 3

ConsoleDsl.scala

trait ConsoleDsl {
  // callers will never know what this is, and that's a good thing
  type A

  def println(s: String): A
  def readLn: A

  def combine(a: A, b: A): A
}

object ConsoleDsl {
  type Aux[T] = ConsoleDsl { type A = T }

  def dsl(using a: ConsoleDsl): a.type = a

  extension [A](a: A)(using ConsoleDsl.Aux[A]) def +(another: A) = summon[ConsoleDsl.Aux[A]].combine(a, another)

}

object Framework {

  def execute(program: (dsl: ConsoleDsl) ?=> dsl.A): String = {

    given ConsoleDsl with {
      type A = Int
      def println(s: String): Int = 0

      def readLn: Int = 1

      def combine(a: Int, b: Int): Int = a + b

    }

    "foo" * program
  }

}

import ConsoleDsl._

@main def hello = println {
  Framework.execute {
    // here, you can use the DSL but you still don't know what A is
    // you can only operate on its values with the operators from the DSL
    dsl.println("a")
      + dsl.println("a")
      + dsl.println("a")
      + dsl.readLn
  }
}