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README.md

Scalaz support for Lift JSON

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 net.liftweb.json.scalaz.JsonScalaz._
scala> import net.liftweb.json._

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 

to

(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("street"), field("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)))

Validation

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("name"), validate[Int]("age") >=> min(18) >=> max(60))

Installation

Add dependency to your SBT project description:

val lift_json_scalaz = "net.liftweb" %% "lift-json-scalaz" % "XXX"

Links

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