Raising Argonaut tuple arity to 23

Question

I'm trying to implement a CodecJson with arity 23. It looks something like this:

implicit def infoCodec: CodecJson[Info] = CodecJson(
    (x: Info) => argonaut.Json(
      "a" -> x.a,
      "b" -> x.b,
      "c" -> x.c,
      ...(a total of 23 fields)
    ),
cursor => for {
      a <- cursor.get("a")
      b <- cursor.get("b")
      c <- cursor.get("c")
      ...(a total of 23 fields)
    }
  )

However, I get type errors on all fields such as:

type mismatch;
  Id|Json
      "a" -> x.a,

How do I convert x.a into Json - and so on for all other fields/types?

Thank you!

Answer 1

EDIT: Scala functions are limited to an arity of 22. So this may just be your problem.

You need to use the "methods" supplied by Argonaut for constructing a JSON: := and ->:. For example:

case class MyCookie(name: String, value: String)
implicit def myCookieCodec = CodecJson((c: MyCookie) =>
    ("name" := c.name) ->:
    ("value" := c.value) ->: 
    jEmptyObject,
    c => for {
        name <- (c --\ "name").as[String]
        value <- (c --\ "value").as[String]
    } yield Cookie(name, value)
)

The ->: implies a "prepend", this means you have to read a chain of ->: calls from right to left. Regarding the example above:

1. Start with an empty object (jEmptyObject)
2. Add a key "value" with the value "c.value" (("value" := c.value))
3. Add a key "name" with the value "c.name" (("name" := c.name))

:= constructs a JSON key-value pair.

I think your decoding part (the second parameter to CodecJson.apply()) should work as intended.