Skip to content
Branch: master
Find file History
Type Name Latest commit message Commit time
Failed to load latest commit information.
src/main/scala/org/json4s remove tailing whitespaces May 20, 2018

Scalaz support for json4s

This project adds a type class to parse JSON:

trait JSON[A] {
  def read(json: JValue): Result[A]
  def write(value: A): JValue

type Result[A] = ValidationNEL[Error, A]

Function 'read' returns an Applicative Functor, enabling parsing in an applicative style.

Simple example

scala> import scalaz._
scala> import Scalaz._
scala> import org.json4s.scalaz.JsonScalaz._
scala> import org.json4s._

scala> case class Address(street: String, zipCode: String)
scala> case class Person(name: String, age: Int, address: Address)

scala> val json = parse(""" {"street": "Manhattan 2", "zip": "00223" } """)
scala> (field[String]("street")(json) |@| field[String]("zip")(json)) { Address }
res0: Success(Address(Manhattan 2,00223))

scala> (field[String]("streets")(json) |@| field[String]("zip")(json)) { Address }
res1: Failure("no such field 'streets'")

Notice the required explicit types when reading fields from JSON. The library comes with helpers which can lift functions with pure values into "parsing context". This works well with Scala's type inferencer:

scala> Address.applyJSON(field("street"), field("zip"))(json)
res2: Success(Address(Manhattan 2,00223))

Function 'applyJSON' above lifts function

(String, String) => Address 


(JValue => Result[String], JValue => Result[String]) => (JValue => Result[Address])

Example which adds a new type class instance

scala> implicit def addrJSONR: JSONR[Address] = Address.applyJSON(field[String]("street"), field[String]("zip"))

scala> val p = JsonParser.parse(""" {"name":"joe","age":34,"address":{"street": "Manhattan 2", "zip": "00223" }} """)
scala> Person.applyJSON(field("name"), field("age"), field("address"))(p)
res0: Success(Person(joe,34,Address(Manhattan 2,00223)))


Applicative style parsing works nicely with validation and data conversion. It is easy to compose transformations with various combinators Scalaz provides. An often used combinator is called a Kleisli composition >=>.

def min(x: Int): Int => Result[Int] = (y: Int) => 
  if (y < x) Fail("min", y + " < " + x) else y.success

def max(x: Int): Int => Result[Int] = (y: Int) => 
  if (y > x) Fail("max", y + " > " + x) else y.success

// Creates a function JValue => Result[Person]
Person.applyJSON(field[String]("name"), validate[Int]("age") >==> min(18) >==> max(60))


Add dependency to your SBT project description:

val json4s_scalaz = "org.json4s" %% "json4s-scalaz" % "XXX"


You can’t perform that action at this time.