Scalaz validation with Argonaut - json

Question

I have a case class and companion object:
case class Person private(name: String, age: Int)
object Person {
def validAge(age: Int) = {
if (age > 18) age.successNel else "Age is under 18".failureNel
}
def validName(name: String) = {
name.successNel
}
def create(name: String, age: Int) = (validAge(age) |#| validName(name))(Person.apply)
}
I want to use Argonaut to parse some JSON and return a Person OR some errors, as a list. So I need to:
Read the JSON from a string, and validate the string is correctly formed
Decode the JSON into a Person, or List of error strings.
I want to return errors in the form of something I can turn into some more JSON like:
{
errors: ["Error1", "Error2"]
}
I first tried using Argonauts decodeValidation method, which returns a Validation[String, X]. Unfortunately, I need a List of errors.
Any suggestions?

Answer 1

I'm adding this as an answer because it's how I'd solve the problem off the top of my head, but I haven't been keeping up closely with Argonaut development for a while, and I'd love to hear that there's a better way. First for the setup, which fixes a few little issues in yours, and adds a condition for validity on names to make the examples later more interesting:
import scalaz._, Scalaz._
case class Person private(name: String, age: Int)
object Person {
def validAge(age: Int): ValidationNel[String, Int] =
if (age > 18) age.successNel else "Age is under 18".failureNel
def validName(name: String): ValidationNel[String, String] =
if (name.size >= 3) name.successNel else "Name too short".failureNel
def create(name: String, age: Int) =
(validName(name) |#| validAge(age))(Person.apply)
}
And then I'd decode the JSON into a (String, Int) pair before creating the Person:
import argonaut._, Argonaut._
def decodePerson(in: String): ValidationNel[String, Person] =
Parse.decodeValidation(in)(
jdecode2L((a: String, b: Int) => (a, b)
)("name", "age")).toValidationNel.flatMap {
case (name, age) => Person.create(name, age)
}
And then:
scala> println(decodePerson("""{ "name": "", "age": 1 }"""))
Failure(NonEmptyList(Name too short, Age is under 18))
Note that this doesn't accumulate errors in more complex cases—e.g. if the value of the name field is a number and age is 1, you'll only get a single error (the name one). Making error accumulation work in cases like that would be considerably more complex.
Relatedly, you'll also see a deprecation warning about the flatMap on Validation, which you can think of as a reminder that accumulation won't happen across the bind. You can tell the compiler that you understand by importing scalaz.Validation.FlatMap._.