Content-Negotiation for http4s

ContentNegotiation.scala

import org.http4s.{MediaType, MediaRange, Request, Response}
import org.http4s.headers.{Accept, MediaRangeAndQValue}
import cats.data.NonEmptyList
import org.http4s.QValue

/*
 * https://tools.ietf.org/html/rfc7231#section-5.3
 */
object ContentNegotiation {

  def mediaTypes[F[_]](
      r: Request[F],
      head: (MediaType, F[Response[F]]),
      tail: (MediaType, F[Response[F]])*
  ): F[Response[F]] = {
    val accepts = r.headers.get(Accept).map(_.values.toList).getOrElse(Nil)
    score(accepts, NonEmptyList(head, tail.toList)).maxByOption(_._1).map(_._2).getOrElse(head._2)
  }

  def score[A](accepts: List[MediaRangeAndQValue], provided: NonEmptyList[(MediaType, A)]): List[(QValue, A)] =
    provided.toList.flatMap {
      case (mediaType, response) =>
        def exact =
          accepts.collectFirst {
            case MediaRangeAndQValue(`mediaType`, qValue) =>
              (qValue, response)
          }

        def tpe =
          accepts.collectFirst {
            case MediaRangeAndQValue(mediaRange, qValue) if mediaType.satisfiedBy(mediaRange) =>
              (qValue, response)
          }

        def range =
          accepts.collectFirst {
            case MediaRangeAndQValue(mediaRange, qValue)
                if mediaRange != MediaRange.`*/*` && mediaRange.satisfiedBy(mediaType) =>
              (qValue, response)
          }

        def wildcard =
          accepts.collectFirst {
            case MediaRangeAndQValue(mediaRange, qValue) if mediaRange == MediaRange.`*/*` =>
              (qValue, response)
          }
        exact orElse tpe orElse range orElse wildcard
    }
}