TypesafeFormatter.scala

object TypesafeFormatter {
  sealed abstract class Dir[A, B] {
    def format : (String => A) => (String => B)
  }
  def l[A](s: String): Dir[A, A] = L[A](s)
  case class L[A](s: String) extends Dir[A, A] {
    def format = {
      (cont: String => A) =>
        (out: String) =>
          cont(out + s)
    }
  }
  def i[A]: Dir[A, Int => A] = I[A]()
  case class I[A]() extends Dir[A, Int => A] {
    def format = {
      (cont: String => A) =>
        (out: String) =>
          (i: Int) => cont(out + i.toString)
    }
  }
  def s[A]: Dir[A, String => A] = S[A]()
  case class S[A]() extends Dir[A, String => A] {
    def format = {
      (cont: String => A) =>
        (out: String) =>
          (s: String) => cont(out + s)
    }
  }
  case class C[A, B, C, D](d1: Dir[A, B], d2: Dir[C, A]) extends Dir[C, B] {
    def format = {
      (cont: String => C) =>
        (out: String) =>
          d1.format.apply(d2.format.apply(cont)).apply(out)
    }
  }

  implicit def cInfix[A, B](d1: Dir[A, B]) = new {
    def ^[C](d2: Dir[C, A]): Dir[C, B] =
      C(d1, d2)
  }

  def format[A](d: Dir[String, A]) = {
    d.format(identity)("")
  }

  def test = {
    import TypesafeFormatter._

    //残念ながら型推論はしてくれない…
    println(format(l[String => String]("hello ") ^ s ^ l("!!"))("world"))
    println(format(l[Int => Int => Int => String]("a:") ^ i ^ l(" b:") ^ i ^ l(" c:") ^ i)(1)(2)(3))
  }
}