circeDecoderNull.scala

// This gist showcase a Circe JSON Decoder for which a null value in a field gets decoded into some 
// arbitrary type, instead of being treated as if it was not present in the request and decoded into None.
// This behaviour is semantically unsound, but so are sometimes the business requirements for application development

//> using scala "2.13.10"
//> using lib "io.circe::circe-core::0.14.6"
//> using lib "io.circe::circe-generic::0.14.6"
//> using lib "io.circe::circe-parser::0.14.6"


import io.circe._, io.circe.parser._
import io.circe.generic.semiauto._
import cats.syntax.all._

object CirceDecoder extends App {
  case class Foo(bar: Option[String], baz: Int)

  object Foo {
    implicit val decoderFoo: Decoder[Foo] = Decoder.instance { c =>
      for {
        bar <- c.downField("bar").focus match {
          case Some(json) if json.isNull => "I was null!".some.asRight
          case Some(json) => json.asString.asRight
          case _ => none[String].asRight
        }
        baz <- c.downField("baz").as[Int]
      } yield Foo(bar, baz)


    }
  }


  val json1 =
    """
{
  "bar" : null,
  "baz" : 1
}
"""

  val json2 =
    """
{
  "bar" : "I was NOT null!",
  "baz" : 1
}
"""


  val json3 =
    """
{
  "baz" : 1
}
"""

  def jsonEqualsTo(json: String, foo: Foo): Boolean = {
    (for {
      parseResult <- parse(json)
      jsonResult <- parseResult.as[Foo]
    } yield jsonResult).contains(foo)
  }

  assert(jsonEqualsTo(json1, Foo(Some("I was null!"),1)))
  assert(jsonEqualsTo(json2, Foo(Some("I was NOT null!"),1)))
  assert(jsonEqualsTo(json3, Foo(none[String],1)))
}