private def createRequestCallback[RequestType: TypeTag, ResponseType: TypeTag]()

in http-server/src/main/scala/com/twitter/finatra/http/internal/routing/CallbackConverter.scala [57:109]
48 lines of code
18 McCabe index (conditional complexity)

  private def createRequestCallback[RequestType: TypeTag, ResponseType: TypeTag](
    callback: RequestType => ResponseType
  ): Request => ResponseType = {
    val manifestRequestType = TypeUtils.asManifest[RequestType]

    manifestRequestType match {
      case request if request == manifest[Request] =>
        callback.asInstanceOf[Request => ResponseType]
      case streamingRequest if runtimeClassEqs[StreamingRequest[Any, _]](streamingRequest) =>
        val streamIdentity = streamingRequest.typeArguments.head
        val streamType = streamingRequest.typeArguments.last
        request: Request =>
          // Cannot assign StreamingRequest result to a value because 2.13 treats these abstractions over HKTs as errors
          streamIdentity match {
            case reader if runtimeClassEqs[Reader[_]](reader) =>
              callback(
                StreamingRequest(jsonStreamParser, request)(FromReader.ReaderIdentity, streamType)
                  .asInstanceOf[RequestType])
            case asyncStream if runtimeClassEqs[AsyncStream[_]](asyncStream) =>
              callback(
                StreamingRequest
                  .fromRequestToAsyncStream(jsonStreamParser, request)(streamType)
                  .asInstanceOf[RequestType])
            case _ =>
              throw new Exception(
                s"Unsupported StreamingRequest type detected as $streamIdentity"
              )
          }
      case asyncStream if runtimeClassEqs[AsyncStream[_]](asyncStream) =>
        val asyncStreamTypeParam = asyncStream.typeArguments.head
        request: Request =>
          val asyncStream = jsonStreamParser.parseArray(request.reader)(asyncStreamTypeParam)
          callback(asyncStream.asInstanceOf[RequestType])
      case reader if runtimeClassEqs[Reader[_]](reader) =>
        val readerTypeParam = reader.typeArguments.head
        request: Request =>
          val reader = jsonStreamParser.parseJson(request.reader)(readerTypeParam)
          callback(reader.asInstanceOf[RequestType])
      case intOrString
          if runtimeClassEqs[Int](intOrString) || runtimeClassEqs[String](intOrString) =>
        // NOTE: callback functions with no input param which return a String are inferred as a RequestType of
        // `Int` by Scala because `StringOps.apply` is a function of Int => Char, other return types
        // infer a `RequestType` of `String`.
        throw new Exception(
          s"Improper callback function RequestType: ${manifestRequestType.runtimeClass}. Controller routes defined with a callback function that has no input parameter or with an incorrectly specified input parameter type are not allowed. " +
            s"Please specify an input parameter in your route callback function of the appropriate type. For more details see: $url"
        )
      case _ =>
        request: Request =>
          val callbackInput = messageBodyManager.read[RequestType](request)
          callback(callbackInput)
    }
  }