A Simple REST DSL Part 6

A big thank you to everyone who attended my talk at ScalaDays

Posted by Iain Hull on June 19, 2014

A big thank you to everyone who attended my talk at ScalaDays. I really enjoyed all your excellent feedback. Here is a blog I was working on before hand.

Before talking about testing I would like to cover some changes I made to extractors and assertions. Previously extractors were simple functions, this is an efficient implementation but the test failures result in generic exceptions and the only clue to the cause is a stack trace. This is not an efficient use of programmers time. The DSL makes tests easy to read and write, so it should make failures easy to diagnose as well as.

Here is an example of the DSL using extractors to create assertions and return values from response. Notice how the returning method can be called after asserting. This works because asserting returns the Response when the assertions succeed (and throws an exception when it fails).

val Jason: Person = ???
val personJson = Json.stringify(Jason)
val EmptyList = List[Person]()

using(_ url "http://api.rest.org/person") { implicit rb =>
  GET asserting (StatusCode === Status.OK, jsonBodyAsList[Person] === EmptyList)
  val id = POST body personJson asserting (StatusCode === Status.Created) returning (Header("X-Person-Id"))
  GET / id asserting (StatusCode === Status.OK, jsonBodyAs[Person] === Jason)
  GET asserting (StatusCode === Status.OK, jsonBodyAsList[Person] === Seq(Jason))
  DELETE / id asserting (StatusCode === Status.OK)
  GET / id asserting (StatusCode === Status.NotFound)
  GET asserting (StatusCode === Status.OK, jsonBodyAsList[Person] === EmptyList)

Previously extractors were defined as simple functions.

type Extractor[T] = Response => T

val statusCode: Extractor[Int] = _.statusCode
val body: Extractor[String] = _.body.get

The extractors core functionality is now described by the ExtractorLike trait. Most extractors are implemented with the Extractor case class.

trait ExtractorLike[+A] {
  def name: String
  def unapply(res: Response): Option[A] = value(res).toOption
  def value(implicit res: Response): Try[A]

case class Extractor[+A](name: String, op: Response => A) extends ExtractorLike[A] {
  override def value(implicit res: Response): Try[A] = {
    Try { op(res) } recoverWith {
      case e =>
        Failure[A](new ExtractorFailedException(
            s"Cannot extract $name from Response: ${e.getMessage}",

  def andThen[B](nextOp: A => B): Extractor[B] = copy(name = name + ".andThen ?", op = op andThen nextOp)

  def as(newName: String) = copy(name = newName)

The name method enables extractors to include a detailed error message when they fail. The unapply method enables extractors to be used in pattern matching and partial functions. The value method extracts the value from the Response, it returns a Try which highlights that this method can fail better than simply throwing an exception.

The Extractor case class implements the value method by calling the op function and wrapping the result in a Try with a detailed exception message. The andThen and as methods enable Extractors to be composed and the result given a new name. These will be covered further down. The most important addition is the name method, which enables the DSL to provide meaningful failure messages when the extracting values from the Response fails.

Here are some examples of how Extractors are defined

val StatusCode = Extractor[Int](
  "StatusCode", r => r.statusCode)

val BodyText = Extractor[String](
  "BodyText", r => r.body.get)

val JsonBody = BodyText andThen Json.parse as "JsonBody"

def jsonBodyAs[T : Reads](path: JsPath): Extractor[T] = {
  JsonBody andThen (json => jsonToValue(json, path)) as "jsonBodyAs"

val BodyAsPerson = jsonBodyAs[Person](JsPath)

val BodyAsName = jsonBodyAs[String](JsPath \ "name")

The implicit class RichResponse adds the returning method to the RequestBuilder. This takes one or more extractors and returns one or more values (using tuples to return multiple values). The returning method is overloaded four times for one to four extractors. This duplicated code could be reduced and made more generic with the Shapeless library, however in this case I didn't think the extra dependency and complication was worth the code reuse. The last thing each method does is call tryValue.get, this will throw a exception for the first extractor that fails. However it is possible for more than one extractor to fail. In this case it would be nice to gather all the failures and throw them in one exception. Again I have left this out but the Scalactic library, which is a part of ScalaTest, contains an Or Type which is like Try but accumulates errors. I might consider using this in the future.

implicit class RichResponse(response: Response) {

  // ...

  def returning[T1](ext1: ExtractorLike[T1])(implicit client: HttpClient): T1 = {

  def returning[T1, T2](ext1: ExtractorLike[T1], ext2: ExtractorLike[T2]): (T1, T2) = {
    val tryValue = for {
      r1 <- ext1.value(response)
      r2 <- ext2.value(response)
    } yield (r1, r2)

The returning can also be used without without calling asserting or execute. This works because there in an implicit conversion from RequestBuilder to RichResponse.

implicit def requestBuilderToRichResponse(builder: RequestBuilder)(implicit client: HttpClient): RichResponse = new RichResponse(builder.execute())

The asserting method takes a list of Assertions

GET / "person" asserting (
  StatusCode === Status.OK,
  BodyAsPersonList === EmptyList)

Assertions are simply functions that take a Response and return an Option[String]. They return None if the assertion passes and a failure message wrapped in a Some if they fail. Assertions are normally created from Extractors using one of the operators defined in RichExtractor. The === and !== operators are defined for all types A and B that are related. Using B :> A enables the value returned from an extractor to be compared with a super class instance as well as subclass instances. The relation operators <, <=, > and >= are defined for any types that have an implicit Ordering.

type Assertion = Function1[Response, Option[String]]

implicit class RichExtractor[A](ext: ExtractorLike[A]) {
  def ===[B >: A](expected: B): Assertion = makeAssertion(_ == expected, expected, "did not equal")
  def !==[B >: A](expected: B): Assertion = makeAssertion(_ != expected, expected, "did equal")

  def <[B >: A](expected: B)(implicit ord: math.Ordering[B]): Assertion = makeAssertion(ord.lt(_, expected), expected, "was not less than")
  def <=[B >: A](expected: B)(implicit ord: math.Ordering[B]): Assertion = makeAssertion(ord.lteq(_, expected), expected, "was not less than or equal")
  def >[B >: A](expected: B)(implicit ord: math.Ordering[B]): Assertion = makeAssertion(ord.gt(_, expected), expected, "was not greater than")
  def >=[B >: A](expected: B)(implicit ord: math.Ordering[B]): Assertion = makeAssertion(ord.gteq(_, expected), expected, "was not greater than or equal")

  def in[B >: A](expectedVals: B*): Assertion = makeAssertion(expectedVals.contains(_), expectedVals.mkString("(", ", ", ")"), "was not in")
  def notIn[B >: A](expectedVals: B*): Assertion = makeAssertion(!expectedVals.contains(_), expectedVals.mkString("(", ", ", ")"), "was in")

  private def makeAssertion[B](pred: A => Boolean, expected: B, text: String): Assertion = { res =>
    val actual = ext.value(res)
    actual match {
      case Success(a) if (!pred(a)) =>
        Some(s"${ext.name}: $a $text $expected")
      case Success(_) =>
      case Failure(e) =>

The asserting method first executes the request to retrieve the response. Then evaluates a sequence of Assertions with this response, if any fail an AssertionFailedException is thrown containing the list of failure messages. Remember Asertions return Some[String] on failure.

implicit class RichRequestBuilder(builder: RequestBuilder) {

  def asserting(assertions: Assertion*)(implicit client: HttpClient): Response = {
    val res = execute()
    val assertionFailures = for {
      a <- assertions
      r <- a(res)
    } yield r
    if (assertionFailures.nonEmpty) {
      throw assertionFailed(assertionFailures)

Extractors can also be used with partial functions. Notice how the post method contains two cases. Extractors can be used for testing API migration. In this case the API can be modified to support the location header, the test continues to pass. When the location header is supported the test uses it, otherwise it defaults to the old behaviour. Later the old behaviour can be removed along with the extra case.

val BodyAsPersonList = jsonBodyAsList[Person]
val BodyAsPerson = jsonBodyAs[Person]
val PersonIdHeader = Header("X-Person-Id")

using(_ url "http://api.rest.org/person") { implicit rb =>
  GET expecting {
    case StatusCode(Status.OK) & BodyAsPersonList(EmptyList) =>
  val id = POST body personJson expecting {
    case StatusCode(Status.Created) & LocationHeader(loc) => extractId(loc)
    case StatusCode(Status.Created) & PersonIdHeader(id) => id
  GET / id expecting {
    case StatusCode(Status.OK) & BodyAsPerson(p) => p should be(Jason)
  GET expecting {
    case StatusCode(Status.OK) & BodyAsPersonList(xp) => xp should be(Seq(Jason))
  DELETE / id expecting {
    case StatusCode(Status.OK) =>
  GET / id expecting {
    case StatusCode(Status.NotFound) =>
  GET expecting {
    case StatusCode(Status.OK) & BodyAsPersonList(EmptyList) =>

Notice how extractors are anded together. This is enabled by the & object whose unapply method takes a Response and returns a Some[(Response, Response)]. This enables both extractors unapply methods to accepts a Response each.

object & {
  def unapply(res: Response): Option[(Response, Response)] = {
    Some((res, res))

The expecting function that supports this usage is very simple.

def expecting[T](func: Response => T)(implicit client: HttpClient): T = {
  val res = execute()

The source code for this post is available on github

If you would like to give me any feedback regarding this post I am happy to discuss anything and everything on Twitter and email. If there is enough interest I will also consider enabling comments on my blog.