This is how Marshalling is defined:

copysource/**
 * Describes one possible option for marshalling a given value.
 */
sealed trait Marshalling[+A] {
  def map[B](f: A => B): Marshalling[B]

  /**
   * Converts this marshalling to an opaque marshalling, i.e. a marshalling result that
   * does not take part in content type negotiation. The given charset is used if this
   * instance is a `WithOpenCharset` marshalling.
   */
  def toOpaque(charset: HttpCharset): Marshalling[A]
}

object Marshalling {

  /**
   * A Marshalling to a specific [[akka.http.scaladsl.model.ContentType]].
   */
  final case class WithFixedContentType[A](
    contentType: ContentType,
    marshal:     () => A) extends Marshalling[A] {
    def map[B](f: A => B): WithFixedContentType[B] = copy(marshal = () => f(marshal()))
    def toOpaque(charset: HttpCharset): Marshalling[A] = Opaque(marshal)
  }

  /**
   * A Marshalling to a specific [[akka.http.scaladsl.model.MediaType]] with a flexible charset.
   */
  final case class WithOpenCharset[A](
    mediaType: MediaType.WithOpenCharset,
    marshal:   HttpCharset => A) extends Marshalling[A] {
    def map[B](f: A => B): WithOpenCharset[B] = copy(marshal = cs => f(marshal(cs)))
    def toOpaque(charset: HttpCharset): Marshalling[A] = Opaque(() => marshal(charset))
  }

  /**
   * A Marshalling to an unknown MediaType and charset.
   * Circumvents content negotiation.
   */
  final case class Opaque[A](marshal: () => A) extends Marshalling[A] {
    def map[B](f: A => B): Opaque[B] = copy(marshal = () => f(marshal()))
    def toOpaque(charset: HttpCharset): Marshalling[A] = this
  }
}

Akka HTTP also defines a number of helpful aliases for the types of marshallers that you’ll likely work with most:

copysourcetype ToEntityMarshaller[T] = Marshaller[T, MessageEntity]
type ToByteStringMarshaller[T] = Marshaller[T, ByteString]
type ToHeadersAndEntityMarshaller[T] = Marshaller[T, (immutable.Seq[HttpHeader], MessageEntity)]
type ToResponseMarshaller[T] = Marshaller[T, HttpResponse]
type ToRequestMarshaller[T] = Marshaller[T, HttpRequest]

• PredefinedToEntityMarshallers
  • Array[Byte]
  • ByteStringByteString
  • Array[Char]
  • String
  • akka.http.scaladsl.model.FormData
  • akka.http.scaladsl.model.MessageEntity
  • T <: akka.http.scaladsl.model.Multipart
• PredefinedToResponseMarshallers
  • T, if a ToEntityMarshaller[T] is available
  • HttpResponseHttpResponse
  • StatusCodeStatusCode
  • (StatusCode, T), if a ToEntityMarshaller[T] is available
  • (Int, T), if a ToEntityMarshaller[T] is available
  • (StatusCode, immutable.Seq[HttpHeader], T), if a ToEntityMarshaller[T] is available
  • (Int, immutable.Seq[HttpHeader], T), if a ToEntityMarshaller[T] is available
• PredefinedToRequestMarshallers
  • HttpRequestHttpRequest
  • UriUri
  • (HttpMethod, Uri, T), if a ToEntityMarshaller[T] is available
  • (HttpMethod, Uri, immutable.Seq[HttpHeader], T), if a ToEntityMarshaller[T] is available
• GenericMarshallers
  • Marshaller<Throwable, T>Marshaller[Throwable, T]
  • Marshaller<Option<A>, B>Marshaller[Option[A], B], if a Marshaller<A, B>Marshaller[A, B] and an EmptyValue[B] is available
  • Marshaller<Either<A1, A2>, B>Marshaller[Either[A1, A2], B], if a Marshaller<A1, B>Marshaller[A1, B] and a Marshaller<A2, B>Marshaller[A2, B] is available
  • Marshaller<Future<A>, B>Marshaller[Future[A], B], if a Marshaller<A, B>Marshaller[A, B] is available
  • Marshaller<Try<A>, B>Marshaller[Try[A], B], if a Marshaller<A, B>Marshaller[A, B] is available

Implicit Resolution

The marshalling infrastructure of Akka HTTP relies on a type-class based approach, which means that MarshallerMarshaller instances from a certain type A to a certain type B have to be available implicitly.

The implicits for most of the predefined marshallers in Akka HTTP are provided through the companion object of the MarshallerMarshaller trait. This means that they are always available and never need to be explicitly imported. Additionally, you can simply “override” them by bringing your own custom version into local scope.

Rather, it should be possible to use one of the convenience construction helpers defined on the MarshallerMarshaller companion:

copysourceobject Marshaller
  extends GenericMarshallers
  with PredefinedToEntityMarshallers
  with PredefinedToResponseMarshallers
  with PredefinedToRequestMarshallers {

  /**
   * Creates a [[Marshaller]] from the given function.
   */
  def apply[A, B](f: ExecutionContext => A => Future[List[Marshalling[B]]]): Marshaller[A, B] =
    new Marshaller[A, B] {
      def apply(value: A)(implicit ec: ExecutionContext) =
        try f(ec)(value)
        catch { case NonFatal(e) => FastFuture.failed(e) }
    }

  /**
   * Helper for creating a [[Marshaller]] using the given function.
   */
  def strict[A, B](f: A => Marshalling[B]): Marshaller[A, B] =
    Marshaller { _ => a => FastFuture.successful(f(a) :: Nil) }

  /**
   * Helper for creating a "super-marshaller" from a number of "sub-marshallers".
   * Content-negotiation determines, which "sub-marshaller" eventually gets to do the job.
   *
   * Please note that all marshallers will actually be invoked in order to get the Marshalling object
   * out of them, and later decide which of the marshallings should be returned. This is by-design,
   * however in ticket as discussed in ticket https://github.com/akka/akka-http/issues/243 it MAY be
   * changed in later versions of Akka HTTP.
   */
  def oneOf[A, B](marshallers: Marshaller[A, B]*): Marshaller[A, B] =
    Marshaller { implicit ec => a => FastFuture.sequence(marshallers.map(_(a))).fast.map(_.flatten.toList) }

  /**
   * Helper for creating a "super-marshaller" from a number of values and a function producing "sub-marshallers"
   * from these values. Content-negotiation determines, which "sub-marshaller" eventually gets to do the job.
   *
   * Please note that all marshallers will actually be invoked in order to get the Marshalling object
   * out of them, and later decide which of the marshallings should be returned. This is by-design,
   * however in ticket as discussed in ticket https://github.com/akka/akka-http/issues/243 it MAY be
   * changed in later versions of Akka HTTP.
   */
  def oneOf[T, A, B](values: T*)(f: T => Marshaller[A, B]): Marshaller[A, B] =
    oneOf(values map f: _*)

  /**
   * Helper for creating a synchronous [[Marshaller]] to content with a fixed charset from the given function.
   */
  def withFixedContentType[A, B](contentType: ContentType)(marshal: A => B): Marshaller[A, B] =
    new Marshaller[A, B] {
      def apply(value: A)(implicit ec: ExecutionContext) =
        try FastFuture.successful {
          Marshalling.WithFixedContentType(contentType, () => marshal(value)) :: Nil
        } catch {
          case NonFatal(e) => FastFuture.failed(e)
        }

      override def compose[C](f: C => A): Marshaller[C, B] =
        Marshaller.withFixedContentType(contentType)(marshal compose f)
    }

  /**
   * Helper for creating a synchronous [[Marshaller]] to content with a negotiable charset from the given function.
   */
  def withOpenCharset[A, B](mediaType: MediaType.WithOpenCharset)(marshal: (A, HttpCharset) => B): Marshaller[A, B] =
    new Marshaller[A, B] {
      def apply(value: A)(implicit ec: ExecutionContext) =
        try FastFuture.successful {
          Marshalling.WithOpenCharset(mediaType, charset => marshal(value, charset)) :: Nil
        } catch {
          case NonFatal(e) => FastFuture.failed(e)
        }

      override def compose[C](f: C => A): Marshaller[C, B] =
        Marshaller.withOpenCharset(mediaType)((c: C, hc: HttpCharset) => marshal(f(c), hc))
    }

  /**
   * Helper for creating a synchronous [[Marshaller]] to non-negotiable content from the given function.
   */
  def opaque[A, B](marshal: A => B): Marshaller[A, B] =
    strict { value => Marshalling.Opaque(() => marshal(value)) }

  /**
   * Helper for creating a [[Marshaller]] combined of the provided `marshal` function
   * and an implicit Marshaller which is able to produce the required final type.
   */
  def combined[A, B, C](marshal: A => B)(implicit m2: Marshaller[B, C]): Marshaller[A, C] =
    Marshaller[A, C] { ec => a => m2.compose(marshal).apply(a)(ec) }
}

However, you can also use the marshalling infrastructure directly if you wish, which can be useful for example in tests. The best entry point for this is the Marshal object, which you can use like this:

copysourceimport scala.concurrent.Await
import scala.concurrent.duration._
import akka.http.scaladsl.marshalling.Marshal
import akka.http.scaladsl.model._

import system.dispatcher // ExecutionContext

val string = "Yeah"
val entityFuture = Marshal(string).to[MessageEntity]
val entity = Await.result(entityFuture, 1.second) // don't block in non-test code!
entity.contentType shouldEqual ContentTypes.`text/plain(UTF-8)`

val errorMsg = "Easy, pal!"
val responseFuture = Marshal(420 -> errorMsg).to[HttpResponse]
val response = Await.result(responseFuture, 1.second) // don't block in non-test code!
response.status shouldEqual StatusCodes.EnhanceYourCalm
response.entity.contentType shouldEqual ContentTypes.`text/plain(UTF-8)`

val request = HttpRequest(headers = List(headers.Accept(MediaTypes.`application/json`)))
val responseText = "Plaintext"
val respFuture = Marshal(responseText).toResponseFor(request) // with content negotiation!
a[Marshal.UnacceptableResponseContentTypeException] should be thrownBy {
  Await.result(respFuture, 1.second) // client requested JSON, we only have text/plain!
}