RESTClient.scala

trait SchemaRegistryAlgebra[F[_]] {
   def getSchemaStringForID(id:Int):F[String]
}

object SchemaRegistryAlgebra{

  private[this] def appendToUrl[E, M[_]: MonadError[?[_], E]](ef:String => E)(url:String Refined Url, suffix:String):M[String Refined Url] =
    refineV[Url]( if (url.value.endsWith("/")) url.value+suffix else url.value + "/" + suffix).fold[M[String Refined Url]](es => MonadError[M, E].raiseError(ef(es)), MonadError[M, E].pure)

  def ioSchemaRegistry(schemaRegistryURL: String Refined Url):SchemaRegistryAlgebra[IO] = new SchemaRegistryAlgebra[IO] {
    val httpClientIO = Http1Client[IO]()
    val cache = Ref.of[IO, Map[Int,String]](Map.empty[Int,String])


    private[this] def appendToUrlIO(url:String Refined Url, suffix:String):IO[String Refined Url] = appendToUrl[Throwable, IO](es => new RuntimeException(es))(url, suffix)

    override def getSchemaStringForID(id: Int): IO[String] = for {
      ref <- cache
      map <- ref.get
      res <- map.get(id).fold(
        for {
          client <- httpClientIO
          url <- appendToUrlIO(schemaRegistryURL, s"schemas/ids/$id")
          resultString <- client.expect[String](url.value)
          out <- ref.modify(currentCache => (currentCache + (id -> resultString), resultString) )
        } yield out
      )(IO.pure)
    } yield res
  }

}