Interpreters of Your Own Design

Unfiltered comes with a small number of interpreters for standard Scala types. We may have left out the one you want (send a pull request), or perhaps you’d like to define interpreters for a type specific to your own application. This is in fact a very good idea!

Conditional Interpreters

A simple way to augment an interpreter is to require a condition for which it is valid. For example, in your application valid identifiers might be non-negative, non-zero integers up to a certain size. Another application might permit a certain set of characters as identifiers.

Rather than attempting to anticipate all the ways you might like to constrain input parameters, Unfiltered leaves it to you to define interpreters using the Scala language. Here, we’ll define a simple one that accepts only even integers.

import unfiltered.request._
import unfiltered.response._
import unfiltered.directives._, Directives._

def badParam(msg: String) =
  BadRequest ~> ResponseString(msg)

val evenInt = data.Conditional[Int](_ % 2 == 0).fail(
  (k, v) => badParam("not even: " + v)
)

As a conditional interpreter of integer, evenInt inputs and outputs an integer, failing with an error if it doesn’t satisfy the condition. To make it work seamlessly with request parameters we may define an implicit integer interpreter.

implicit val intValue =
  data.as.String ~> data.as.Int.fail(
    (k, v) => badParam("not an int: " + v)
  )

Then it’s just a matter of using evenInt like any other interpreter.

import unfiltered.jetty.SocketPortBinding

val binding = SocketPortBinding(host = "localhost", port = 8080)

unfiltered.jetty.Server.portBinding(binding).plan(
  unfiltered.filter.Planify { Directive.Intent {
    case Path("/") =>
      for {
        even <- evenInt named "even"
      } yield ResponseString(
        even + "\n"
      )
  } }
).run()

Fallible Interpreters

Conditional interpreters are great for adding application-specific constraints to existing interpreters, but often you’ll need to create interpreters for your own types. These are known as fallible interpreters, under the assumption that any conversion may fail.

You can make a fallible interpreter to any type. We’ll demonstrate with a simple typed data store.

case class Tool(name: String)
val toolStore = Map(
  1 -> Tool("Rock"),
  2 -> Tool("Paper"),
  3 -> Tool("Scissors")
)

val asTool = data.Fallible[Int,Tool](toolStore.get)

implicit def implyTool =
  data.as.String ~> data.as.Int ~> asTool.fail(
    (k, v) => badParam(s"'$v' is not a valid tool identifier")
  )

The data.Fallible case class takes a function from its input type to an option of its output type; Map#get fits the bill perfectly. Then we defined a complete, error-capable interpreter so that it’s easy and clean to turn input parameters into tools.

unfiltered.jetty.Server.portBinding(binding).plan(
  unfiltered.filter.Planify { Directive.Intent {
    case Path("/") =>
      for {
        tool <- data.as.Option[Tool] named "id"
      } yield ResponseString(
        tool + "\n"
      )
  } }
).run()

Let’s see how it works:

curl http://127.0.0.1:8080/  -d id=3
Some(Tool(Scissors))

With a parameter directive you can interpret the input into any type you like. We used an in-memory map, but it could just as easily be an entity loaded from external storage.

The source code for this page can be found here.