Within the Parameters

Extracting Params

Requests are commonly accompanied by named parameters, either in the query string of the URL or in the body of a POST. Unfiltered supports access to these parameters with the Params extractor.

sourceimport unfiltered.request._
import unfiltered.response._
val pEcho = unfiltered.filter.Planify {
  case Params(params) =>

The type of params extracted is Map[String, Seq[String]], with a default value of Seq.empty. With this interface, it is always safe to apply the map. But keep in mind that parameters may be specified with no value, and may occur multiple times. The Params extractor returns empty strings for named parameters with no value, as many times in the sequence as they occurred in the request.

For example, the query string ?test&test=3&test produces the sequence of strings "", "3", "". You can check this yourself by querying the plan defined above:


Routing by Parameter

While the Params extractor is useful for accessing parameters, it doesn’t provide any control flow to an intent partial function.

If you want to route requests based on the parameters present, you’ll need to nest a custom extractor inside Params. For this Unfiltered provides a Params.Extract base class:

sourceobject Test extends Params.Extract("test", Params.first)

The above extractor will match on the first parameter named “test” in a request. If no parameters are named test, the pattern does not match. However, a named parameter with no value would match. We can exclude this possibility with a richer definition.

sourceobject NonEmptyTest extends Params.Extract(
  Params.first ~> Params.nonempty

The second parameter of the Params.Extract constructor is a function Seq[String] => Option[B], with B being the type extracted. The values first and nonempty are functions defined in the Params object that may be chained together with ~> (as with response functions).

Typically the chain begins with first, to require at least one parameter of the given name and discard the rest. Subsequent functions may require a nonempty value as above, or produce a trimmed string, or an int value from the string.

When a parameter transformation function fails, None is produced and the extractor does not match for the request. Knowing this, you can write your own transformation functions using map and filter.

sourceobject NotShortTest extends Params.Extract(
  Params.first ~> { p: Option[String] =>
    p.filter { _.length > 4 }

There’s also a convenience function to simplify the definition of transformation predicates.

sourceobject NotShortTest2 extends Params.Extract(
  Params.first ~> Params.pred { _.length > 4 }

Try it all out in this server, which returns 404 unless provided with a “pos” parameter that is a positive integer, and “neg” that is negative.

sourceobject Pos extends Params.Extract(
  Params.first ~> Params.int ~>
    Params.pred { _ > 0 }
object Neg extends Params.Extract(
  Params.first ~> Params.int ~>
    Params.pred { _ < 0 }
val intEcho = unfiltered.filter.Planify {
  case Params(Pos(pos) & Neg(neg)) =>
    ResponseString(s"${pos} ${neg}")

The & extractor matches when the extractor to its left and right match, in this case to require multiple parameters.

The source code for this page can be found here.