Stringified object representations don't preserve field order

[longcao]

Currently, stringified io.circe.Json doesn't appear to preserve field order. This doesn't matter for machines but it would greatly help human readability.

scala> import io.circe._, io.circe.generic.auto._, io.circe.jawn._, io.circe.syntax._
import io.circe._
import io.circe.generic.auto._
import io.circe.jawn._
import io.circe.syntax._

scala> case class MyClass(a: Int, b: String, c: List[String], d: Boolean)
defined class MyClass

scala> val m = MyClass(5, "asdf", List("zxcv", "qwer", "hjkl"), false)
m: MyClass = MyClass(5,asdf,List(zxcv, qwer, hjkl),false)

scala> m.asJson
res0: io.circe.Json =
{
  "d" : false,
  "c" : [
    "zxcv",
    "qwer",
    "hjkl"
  ],
  "b" : "asdf",
  "a" : 5
}

[alexarchambault]

@longcao Order is preserved internally, it's just that the default printers don't preserve it in their output, see https://github.com/travisbrown/circe/blob/master/core/shared/src/main/scala/io/circe/Printer.scala#L215 - it's the same as in argonaut.

The following gives back the right order for me:

Printer.spaces2.copy(preserveOrder = true).pretty(m.asJson)

giving

res1: String = """
{
  "a" : 5,
  "b" : "asdf",
  "c" : [
    "zxcv",
    "qwer",
    "hjkl"
  ],
  "d" : false
}
"""

[travisbrown]

Fixed by #89—thanks @longcao, @alexarchambault!