title: How do I send multiple messages as my source
body: <p>I'm just trying out this sample stream that currently has a single TextMessage as a source:</p>
  <pre><code> // print each incoming strict text message
      val printSink: Sink[Message, Future[Done]] =
        Sink.foreach {
          case message: TextMessage.Strict =&gt;
            println(message.text)
          case _ =&gt;
          // ignore other message types
        }
  
      val helloSource: Source[Message, NotUsed] =
        Source.single(TextMessage(&quot;hello world!&quot;))
  
      // the Future[Done] is the materialized value of Sink.foreach
      // and it is completed when the stream completes
      val flow: Flow[Message, Message, Future[Done]] =
        Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)
  </code></pre>
  <p>I want to send 2 messages, so I tried this:</p>
  <pre><code>val source1 = Source.single(TextMessage(&quot;hello&quot;))
  val source2 = Source.single(TextMessage(&quot;world&quot;))
  
  val helloSource: Source[Message, NotUsed] =
    Source.combine(source2)
  </code></pre>
  <p>But I get this error:</p>
  <pre><code> polymorphic expression cannot be instantiated to expected type;
  [error]  found   : [U](strategy: Int =&gt; akka.stream.Graph[akka.stream.UniformFanInShape[akka.http.scaladsl.model.ws.TextMessage.Strict,U],akka.NotUsed]): akka.stream.scaladsl.Source[U,akka.NotUsed]
  [error]  required: akka.stream.scaladsl.Source[akka.http.scaladsl.model.ws.Message,akka.NotUsed]
  [error]       Source.combine(source1, source2)
  [error]                     ^
  [error] one error found
  </code></pre>
  <p>What exactly should I be doing instead?</p>