I needed code to serve static files from an Akka HTTP server. I wanted to use fs2 to read the static file. Michael Pilquist recommended using streamz to convert from an fs2 Task to an Akka Source. This is what I came up with. (It does actually work.)

StaticFile.scala

object StaticFile {

  // Various necessary imports. Notes:
  //
  // 1. fs2 is necessary. See https://github.com/functional-streams-for-scala/fs2
  // 2. streamz is necessary. See https://github.com/krasserm/streamz
  // 3. Apache Tika is used to infer MIME types from file names, because it's more reliable and
  //    fully-featured than using java.nio.file.Files.probeContentType().
  //
  // If using SBT, you'll want these library dependencies and resolvers:
  //
  // resolvers += "krasserm at bintray" at "http://dl.bintray.com/krasserm/maven"
  // libraryDependencies ++= Seq(
  //   "com.typesafe.akka"   %% "akka-http"            % "10.0.0",
  //   "com.typesafe.akka"   %% "akka-http-spray-json" % "10.0.0",
  //   "com.github.krasserm" %% "streamz-akka-stream"  % "0.5.1",
  //   "org.apache.tika"     %  "tika-core"            % "1.14",
  //   "co.fs2"              %% "fs2-core"             % "0.9.2",
  //   "co.fs2"              %% "fs2-io"               % "0.9.2"
  // )
  import akka.NotUsed
  import akka.http.scaladsl.server.Directives._
  import akka.stream.scaladsl.{Source => AkkaSource}
  import akka.http.scaladsl.model._
  import akka.util.ByteString
  import akka.stream.{Graph, SourceShape}
  import streamz.akka.stream._
  import fs2._
  import org.apache.tika.Tika
  import scala.concurrent.{ExecutionContext, Future}
  import scala.util.control.NonFatal
  import java.nio.file.{AccessDeniedException, NoSuchFileException, Paths}

  val BaseDir = "/path/to/static/base/dir"

  val tika = new Tika

  /** Serve up a static file, using fs2. Here's how to use this function inside
    * an Akka HTTP route:
    *
    * {{{
    * pathPrefix("static" / Remaining) { urlPath =>
    *   get {
    *     complete(StaticFile.serve(s"static/$urlPath"))
    *   }
    * } ~
    * path("/") {
    *   get {
    *     complete(StaticFile.serve("index.html"))
    *   }
    * } ~ // etc
    * }}}
    *
    * @param urlPath  the URL path, as extracted by Akka.
    * @param ctx      the execution context (presumably from Akka, e.g., `ActorSystem("server").dispatcher`)
    *
    * @return An Akka `HttpResponse`, wrapped in a `Future`.
    */
  def serve(urlPath: String)(implicit ctx: ExecutionContext): Future[HttpResponse] = {
    Future {
      // In this case, the base directory is hard-coded, and we need
      // to use it to determine the actual on-disk path. There are other
      // approaches one could use (e.g., reading the base directory from
      // a config, allowing an environment variable override, etc.).
      val pathPieces = BaseDir :+ urlPath.split("/")
      val path = Paths.get(pathPieces.mkString(java.io.File.separator))

      // Use Apache Tika to get the MIME type from the path. This isn't
      // wrapped in an Option because it always seems to return *something*.
      val mime = tika.detect(path)

      // Parse the resulting MIME type string into an Akka HTTP ContentType.
      ContentType.parse(mime) match {
        case Right(contentType) =>
          // We got a valid MIME type, so it's time to try to read the file.
          // fs2 will throw exceptions on error; those are explicitly handled
          // in the Future's recover() block, below.
        
          // Note: I have added explicit types for clarity. Obviously, you can
          // leave them off and let the compiler infer them.
          val source: Graph[SourceShape[ByteString], NotUsed] =
            io.file.                            // We want to use fs2 to read a file.
               readAll[Task](path, 8192).       // Read the whole file as an effectful stream, in 8K chunks
               chunks.                          // Get the chunks
               map { ch: NonEmptyChunk[Byte] =>
                 // We need to map the fs2 chunks into Akka ByteString objects, since
                 // that's what Akka expects in its streams.
                 ByteString(ch.toArray)
               }.
               // Convert the FS2 Stream[A, ByteString] to an Akka Stream
               // Graph[SourceShape[ByteString], NotUsed]. As Martin Krasser
               // notes, the actual FS2 -> Akka Stream conversion happens here,
               // via streamz implicits.
               toSource
        
          // Map the Akka Stream Graph object into an Akka Source.
          val akkaSource: AkkaSource[ByteString, NotUsed] = AkkaSource.fromGraph(source)
        
          // We can return the Akka Source wrapped in the HttpResponse. Note that
          // NO data has been read from the file yet! When Akka HTTP materializes its Source,
          // to stream the bytes up to the browser, it will implicitly materialize the
          // underlying fs2 stream. The "edge" here is inside Akka HTTP's browser-delivery
          // logic.
          HttpResponse(StatusCodes.OK, entity = HttpEntity(contentType, akkaSource))

        case Left(errors) =>
          // We failed to determine the MIME type.
          HttpResponse(StatusCodes.InternalServerError, entity = s"Can't determine MIME type: ${errors.mkString(" ")}")
      }
    }
    .recover {
      // Here's where we handle any errors that occur. Note that fs2 I/O errors will
      // throw java.nio exceptions. We map them explicitly into HTTP result codes.
      case e: NoSuchFileException =>
        HttpResponse(StatusCodes.NotFound, entity = "Not found.")
      case e: AccessDeniedException =>
        HttpResponse(StatusCodes.Forbidden, entity = "Permission denied.")
      case NonFatal(e) =>
        HttpResponse(StatusCodes.InternalServerError, entity = s"Read failure: $e")
    }
  }
}

This Gist incorporates suggestions from both @mpilquist and @krasserm.

Hi,
Lines 65 & 66 seem a bit odd. From the REPL of Scala 2.12.6, there is the following:

==========================================================
Welcome to Scala 2.12.6 (OpenJDK 64-Bit Server VM, Java 1.8.0_171).
Type in expressions for evaluation. Or try :help.

scala> import java.nio.file.{AccessDeniedException, NoSuchFileException, Paths}
import java.nio.file.{AccessDeniedException, NoSuchFileException, Paths}

scala> val urlPath = "index.html"
urlPath: String = index.html

scala> val BaseDir = "/path/to/some/dir"
BaseDir: String = /path/to/some/dir

scala> val pathPieces = BaseDir :+ urlPath.split("/")
pathPieces: scala.collection.immutable.IndexedSeq[Any] = Vector(/, p, a, t, h, /, t, o, /, s, o, m, e, /, d, i, r, Array(index.html))

scala> val path = Paths.get(pathPieces.mkString(java.io.File.separator))
path: java.nio.file.Path = /p/a/t/h/t/o/s/o/m/e/d/i/r/[Ljava.lang.String;@2bb717d7

The path does not look right. Am I missing something here?

Thanks.