Custom Directives

map and tmap

If the Directive is a single-value Directive, the map method allows for simple transformations:

val textParam: Directive1[String] =
  parameter("text".as[String])

val lengthDirective: Directive1[Int] =
  textParam.map(text => text.length)

// tests:
Get("/?text=abcdefg") ~> lengthDirective(x => complete(x.toString)) ~> check {
  responseAs[String] === "7"
}

One example of a predefined directive relying on map is the optionalHeaderValue directive.

The tmap modifier has this signature (somewhat simplified):

def tmap[R](f: L ⇒ R): Directive[Out]

It can be used to transform the Tuple of extractions into another Tuple. The number and/or types of the extractions can be changed arbitrarily. For example if R is Tuple2[A, B] then the result will be a Directive[(A, B)]. Here is a somewhat contrived example:

val twoIntParameters: Directive[(Int, Int)] =
  parameters(("a".as[Int], "b".as[Int]))

val myDirective: Directive1[String] =
  twoIntParameters.tmap {
    case (a, b) => (a + b).toString
  }

// tests:
Get("/?a=2&b=5") ~> myDirective(x => complete(x)) ~> check {
  responseAs[String] === "7"
}

flatMap and tflatMap

With map and tmap you can transform the values a directive extracts but you cannot change the “extracting” nature of the directive. For example, if you have a directive extracting an Int you can use map to turn it into a directive that extracts that Int and doubles it, but you cannot transform it into a directive, that doubles all positive Int values and rejects all others.

In order to do the latter you need flatMap or tflatMap. The tflatMap modifier has this signature:

def tflatMap[R: Tuple](f: L ⇒ Directive[R]): Directive[R]

The given function produces a new directive depending on the Tuple of extractions of the underlying one. As in the case of map and tmap there is also a single-value variant called flatMap, which simplifies the operation for Directives only extracting one single value.

Here is the (contrived) example from above, which doubles positive Int values and rejects all others:

val intParameter: Directive1[Int] = parameter("a".as[Int])

val myDirective: Directive1[Int] =
  intParameter.flatMap {
    case a if a > 0 => provide(2 * a)
    case _          => reject
  }

// tests:
Get("/?a=21") ~> myDirective(i => complete(i.toString)) ~> check {
  responseAs[String] === "42"
}
Get("/?a=-18") ~> myDirective(i => complete(i.toString)) ~> check {
  handled === false
}

A common pattern that relies on flatMap is to first extract a value from the RequestContext with the extract directive and then flatMap with some kind of filtering logic. For example, this is the implementation of the method directive:

The explicit type parameter [Unit] on the flatMap i`s needed in this case because the result of the flatMap is directly concatenated with the cancelAllRejections directive, thereby preventing “outside-in” inference of the type parameter value.

require and trequire

The require modifier transforms a single-extraction directive into a directive without extractions, which filters the requests according the a predicate function. All requests, for which the predicate is false are rejected, all others pass unchanged.

The signature of require is this:

def require(predicate: T ⇒ Boolean, rejections: Rejection*): Directive0

One example of a predefined directive relying on require is the first overload of the host directive:

You can only call require on single-extraction directives. The trequire modifier is the more general variant, which takes a predicate of type Tuple => Boolean. It can therefore also be used on directives with several extractions.

recover and recoverPF

The recover modifier allows you “catch” rejections produced by the underlying directive and, instead of rejecting, produce an alternative directive with the same type(s) of extractions.

The signature of recover is this:

def recover[R >: L: Tuple](recovery: Seq[Rejection] ⇒ Directive[R]): Directive[R] =

In many cases the very similar recoverPF modifier might be little bit easier to use since it doesn’t require the handling of all rejections:

def recoverPF[R >: L: Tuple](
  recovery: PartialFunction[Seq[Rejection], Directive[R]]): Directive[R]

One example of a predefined directive relying recoverPF is the optionalHeaderValue directive: