Ordinary Akka HTTP DSL requires additional brackets to perform directives. For example, the following extractParametersWithBrackets extracts query parameters p1 and p2 with the help of the directive akka.http.scaladsl.server.Directives.parameters.

import akka.http.scaladsl.server.Directives._
def extractParametersWithBrackets = {
  get {
    parameters("p1", "p2") { (p1, p2) =>
      complete(s"$p1, $p2!")
    }
  }
}

Get("/?p1=Hello&p2=World") ~> extractParametersWithBrackets ~> check {
  responseAs[String] should be("Hello, World!")
}

When you have lots of directives, you will experience the callback hell problem, as the directives become heavily nested scopes and the code become too complicated to read. Those brackets can be avoided with the help of this TApply keyword:

def extractParametersWithoutBrackets = {
  get {
    val (p1, p2) = !TApply(parameters("p1", "p2"))
    complete(s"$p1, $p2!")
  }
}

Get("/?p1=Hello&p2=World") ~> extractParametersWithoutBrackets ~> check {
  responseAs[String] should be("Hello, World!")
}

In addition, this TApply keyword can be used together with other keywords. For example, you can use com.thoughtworks.dsl.keywords.Using to automatically manage resources.

import akka.http.scaladsl.server.Directives._
import com.thoughtworks.dsl.keywords.Using
import com.thoughtworks.dsl.keywords.akka.http.TApply
import java.nio.file.Files.newDirectoryStream
import java.nio.file.Paths
import scala.collection.JavaConverters._

def currentDirectoryRoute = pathPrefix("current-directory") {
  get {
    val glob = !TApply(parameters("glob"))
    val currentDirectory = !Using(newDirectoryStream(Paths.get(""), glob))
    path("file-names") {
      complete(currentDirectory.iterator.asScala.map(_.toString).mkString(","))
    } ~ path("number-of-files") {
      complete(currentDirectory.iterator.asScala.size.toString)
    }
  }
}

With the help of the Using keyword, the currentDirectoryRoute will open the current directory according to the glob pattern extract from the query parameter, and automatically close the directory after the HTTP request is processed completely.

Get("/current-directory/file-names?glob=*.md") ~> currentDirectoryRoute ~> check {
  responseAs[String] should be("README.md")
}
Get("/current-directory/file-names?glob=*.*") ~> currentDirectoryRoute ~> check {
  responseAs[String].split(',') should (contain("secret.sbt") and contain("build.sbt") and contain("README.md"))
}
Get("/current-directory/number-of-files?glob=*.sbt") ~> currentDirectoryRoute ~> check {
  responseAs[String] should be("2")
}