composingConverters.scala

sealed trait Json
case class JsonString(value: String) extends Json

sealed trait Yaml
case class YamlString(value: String) extends Yaml

object Symbols {
  type |>[From, To] = Converter[From, To]
  type <|[To, From] = Converter[From, To]
}

import Symbols._

trait Converter[From, To] {
  def apply(from: From): Option[To]
}

object Converter {
  def apply[From, To](from: From)(implicit converter: From |> To): Option[To] = converter(from)

  def apply[FromA, FromB, ToA, ToB](f: FromA => FromB)
  (implicit cfa: FromA <| ToA, cfb: FromB |> ToB): ToA => Option[ToB] =
    toa => cfa(toa).map(f).flatMap(cfb.apply)

  implicit def compose[From, Mid, To](implicit step1: From |> Mid, step2: Mid |> To): From |> To =
    from => step1(from).flatMap(step2.apply)
}

object Main extends App {
  implicit val stringToJsonString: String |> JsonString = from => Some(JsonString(from))
  implicit val jsonStringToString: JsonString |> String = from => Some(from.value)

  implicit val stringToYamlString: String |> YamlString = from => Some(YamlString(from))
  implicit val yamlStringToString: YamlString |> String = from => Some(from.value)

  val jstring = JsonString("meow")
  val optYamlString = Converter[JsonString, YamlString](jstring)

  val f: String => String = s => s + "meow"
  val jf: JsonString => Option[JsonString] = Converter[String, String, JsonString, JsonString](f)

  println(optYamlString)
  println(jf(JsonString("blah")))
}