A Map[String, String] decoder for Circe

CirceMapDecoder.scala

// warning: barely tested

import io.circe.Decoder.Result
import io.circe._
import io.circe.generic.extras._

object CirceMapDecoder {
  type KeyVal = Map[String, String]

  // using the expansion may be necessary for Circe to detect it correctly
  implicit val decodeMap: Decoder[Map[String, String]] = new Decoder[Map[String, String]] {
    override def apply(c: HCursor): Decoder.Result[KeyVal] = {

      def extractValueForKeyInto(res: Decoder.Result[KeyVal], key: String): Decoder.Result[KeyVal] = {
        res.right.flatMap((map: KeyVal) => {
          c.downField(key).as[String].fold(
            Left(_),
            value => Right(map + (key -> value))
          )
        })
      }

      def emptyMapResult: Decoder.Result[KeyVal] = Right(Map[String, String]().empty)

      def withKeys(keys: Iterable[String]): Decoder.Result[KeyVal] = keys.foldLeft(emptyMapResult)(extractValueForKeyInto)

      c.keys.fold[Decoder.Result[KeyVal]](ifEmpty = emptyMapResult)(withKeys)
    }
}