Extensions: Supporting new types

The validation API is designed to be easily extensible. Supporting new types is just a matter of providing the appropriate set of Rules and Writes.

In this documentation, we'll study the implementation of the Json support. All extensions are to be defined in a similar fashion. The total amount of code needed is rather small, but there're best practices you need to follow.


The first step is to define what we call primitive rules. Primitive rules is a set of rules on which you could build any complex validation.

The base of all Rules is the capacity to extract a subset of some input data.

For the type JsValue, we need to be able to extract a JsValue at a given Path:

scala> import jto.validation._
import jto.validation._

scala> import play.api.libs.json.{KeyPathNode => JSKeyPathNode, IdxPathNode => JIdxPathNode, _}
import play.api.libs.json.{KeyPathNode=>JSKeyPathNode, IdxPathNode=>JIdxPathNode, _}

scala> object Ex1 {
     |   def pathToJsPath(p: Path) =
     |     play.api.libs.json.JsPath(p.path.map{
     |       case KeyPathNode(key) => JSKeyPathNode(key)
     |       case IdxPathNode(i) => JIdxPathNode(i)
     |     })
     |   implicit def pickInJson(p: Path): Rule[JsValue, JsValue] =
     |     Rule[JsValue, JsValue] { json =>
     |       pathToJsPath(p)(json) match {
     |         case Nil => Invalid(Seq(Path -> Seq(ValidationError("error.required"))))
     |         case js :: _ => Valid(js)
     |       }
     |     }
     | }
defined object Ex1

Now we are able to do this:

scala> {
     |   import Ex1._
     |   val js = Json.obj(
     |   "field1" -> "alpha",
     |   "field2" -> 123L,
     |   "field3" -> Json.obj(
     |     "field31" -> "beta",
     |     "field32"-> 345))
     |   val pick: Rule[JsValue, JsValue] = From[JsValue] { __ =>
     |     (__ \ "field2").read[JsValue]
     |   }
     |   pick.validate(js)
     | }
res0: jto.validation.VA[play.api.libs.json.JsValue] = Valid(123)

Which is nice, but is would be much more convenient if we could extract that value as an Int.

One solution is to write the following method:

implicit def pickIntInJson[O](p: Path): Rule[JsValue, JsValue] = ???

But we would end up copying 90% of the code we already wrote. Instead of doing so, we're going to make pickInJson a bit smarter by adding an implicit parameter:

implicit def pickInJson[O](p: Path)(implicit r: Rule[JsValue, O]): Rule[JsValue, O] =
    Rule[JsValue, JsValue] { json =>
      pathToJsPath(p)(json) match {
        case Nil => Invalid(Seq(Path -> Seq(ValidationError("error.required"))))
        case js :: _ => Valid(js)

The now all we have to do is to write a Rule[JsValue, O], and we automatically get the Path => Rule[JsValue, O] we're interested in. The rest is just a matter of defining all the primitives rules, for example:

scala> def jsonAs[T](f: PartialFunction[JsValue, Validated[Seq[ValidationError], T]])(args: Any*) =
     |   Rule.fromMapping[JsValue, T](
     |     f.orElse{ case j => Invalid(Seq(ValidationError("validation.invalid", args: _*)))
     |   })
jsonAs: [T](f: PartialFunction[play.api.libs.json.JsValue,jto.validation.Validated[Seq[jto.validation.ValidationError],T]])(args: Any*)jto.validation.Rule[play.api.libs.json.JsValue,T]

scala> def stringRule = jsonAs[String] {
     |   case JsString(v) => Valid(v)
     | }("String")
stringRule: jto.validation.Rule[play.api.libs.json.JsValue,String]

scala> def booleanRule = jsonAs[Boolean]{
     |   case JsBoolean(v) => Valid(v)
     | }("Boolean")
booleanRule: jto.validation.Rule[play.api.libs.json.JsValue,Boolean]

The types you generally want to support natively are:

  • String
  • Boolean
  • Int
  • Short
  • Long
  • Float
  • Double
  • java BigDecimal
  • scala BigDecimal

Higher order Rules

Supporting primitives is nice, but not enough. Users are going to deal with Seq and Option. We need to support those types too.


What we want to do is to implement a function that takes a Path => Rule[JsValue, O], an lift it into a Path => Rule[JsValue, Option[O]] for any type O. The reason we're working on the fully defined Path => Rule[JsValue, O] and not just Rule[JsValue, O] is because a non-existent Path must be validated as a Valid(None). If we were to use pickInJson on a Rule[JsValue, Option[O]], we would end up with an Invalid in the case of non-existing Path.

The jto.validation.DefaultRules[I] traits provides a helper for building the desired method. It's signature is:

protected def opt[J, O](r: => Rule[J, O], noneValues: Rule[I, I]*)(implicit pick: Path => Rule[I, I], coerce: Rule[I, J]): Path = Rule[I, O]
  • noneValues is a List of all the values we should consider to be None. For Json that would be JsNull.
  • pick is an extractor. It's going to extract a subtree.
  • coerce is a type conversion Rule
  • r is a Rule to be applied to the data if they are found

All you have to do is to use this method to implement a specialized version for your type. For example it's defined this way for Json:

def optionR[J, O](r: => Rule[J, O], noneValues: Rule[JsValue, JsValue]*)(implicit pick: Path => Rule[JsValue, JsValue], coerce: Rule[JsValue, J]): Path => Rule[JsValue, Option[O]]
    = super.opt[J, O](r, (jsNull.map(n => n: JsValue) +: noneValues):_*)

Basically it's just the same, but we are now only supporting JsValue. We are also adding JsNull is the list of None-ish values.

Despite the type signature funkiness, this function is actually really simple to use:

val maybeEmail: Rule[JsValue, Option[String]] = From[JsValue] { __ =>
  import jto.validation.playjson.Rules._
  (__ \ "email").read(optionR(email))
scala> maybeEmail.validate(Json.obj("email" -> "foo@bar.com"))
res1: jto.validation.VA[Option[String]] = Valid(Some(foo@bar.com))

scala> maybeEmail.validate(Json.obj("email" -> "baam!"))
res2: jto.validation.VA[Option[String]] = Invalid(List((/email,List(ValidationError(List(error.email),WrappedArray())))))

scala> maybeEmail.validate(Json.obj("email" -> JsNull))
res3: jto.validation.VA[Option[String]] = Valid(None)

scala> maybeEmail.validate(Json.obj())
res4: jto.validation.VA[Option[String]] = Valid(None)

Alright, so now we can explicitly define rules for optional data.

But what if we write (__ \ "age").read[Option[Int]]? It does not compile! We need to define an implicit rule for that:

implicit def option[O](p: Path)(implicit pick: Path => Rule[JsValue, JsValue], coerce: Rule[JsValue, O]): Rule[JsValue, Option[O]] =
    option(Rule.zero[O])(pick, coerce)(p)
val maybeAge: Rule[JsValue, Option[Int]] = From[JsValue] { __ =>
  import jto.validation.playjson.Rules._
  (__ \ "age").read[Option[Int]]


It's very important that every Rule is completely lazily evaluated . The reason for that is that you may be validating recursive types:

scala> case class RecUser(name: String, friends: Seq[RecUser] = Nil)
defined class RecUser

scala> val u = RecUser(
     |   "bob",
     |   Seq(RecUser("tom")))
u: RecUser = RecUser(bob,List(RecUser(tom,List())))

scala> lazy val w: Rule[JsValue, RecUser] = From[JsValue] { __ =>
     |   import jto.validation.playjson.Rules._
     |   ((__ \ "name").read[String] ~
     |    (__ \ "friends").read(seqR(w)))(RecUser.apply) // !!! recursive rule definition
     | }
w: jto.validation.Rule[play.api.libs.json.JsValue,RecUser] = <lazy>


Writes are implemented in a similar fashion, but a generally easier to implement. You start by defining a function for writing at a given path:

scala> {
     |   implicit def writeJson[I](path: Path)(implicit w: Write[I, JsValue]): Write[I, JsObject] = ???
     | }

And you then define all the primitive writes:

scala> {
     |   implicit def anyval[T <: AnyVal] = ???
     | }


In order to be able to use writes combinators, you also need to create an implementation of Monoid for your output type. For example, to create a complex write of JsObject, we had to implement a Monoid[JsObject]:

scala> {
     |   import cats.Monoid
     |   implicit def jsonMonoid = new Monoid[JsObject] {
     |     def combine(a1: JsObject, a2: JsObject) = a1 deepMerge a2
     |     def empty = Json.obj()
     |   }
     | }

from there you're able to create complex writes like:

import jto.validation._
import play.api.libs.json._
import scala.Function.unlift

case class User(
  name: String,
  age: Int,
  email: Option[String],
  isAlive: Boolean)

val userWrite = To[JsObject] { __ =>
  import jto.validation.playjson.Writes._
  ((__ \ "name").write[String] ~
   (__ \ "age").write[Int] ~
   (__ \ "email").write[Option[String]] ~
   (__ \ "isAlive").write[Boolean])(unlift(User.unapply))


We highly recommend you to test your rules as much as possible. There're a few tricky cases you need to handle properly. You should port the tests in RulesSpec.scala and use them on your rules.

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