Play JSON Reads/Writes with single-parameter case classes

Question

This creates a Writes for a case class

import play.api.libs.json._
import play.api.libs.functional.syntax._

case class A(a: String, b: String, c: String)
(JsPath.write[String] and
    JsPath.write[String] and
    JsPath.write[String])(unlift(A.unapply))

This can be extended to work for 2, 3, 4, 5, 6, etc. parameters...but not 1.

case class B(a: String)
(JsPath.write[String])(unlift(B.unapply))

Compiler error:

error: overloaded method value write with alternatives:
  (t: String)(implicit w:  play.api.libs.json.Writes[String])play.api.libs.json.OWrites[play.api.libs.json.JsValue] <and>
  (implicit w: play.api.libs.json.Writes[String])play.api.libs.json.OWrites[String]
  cannot be applied to (B => String)
              (JsPath.write[String])(unlift(B.unapply))
                           ^

A similar problem happens for Reads.

How can I get Reads and Writes for single-parameter case clases?

Answer 1

Like Travis said:

1. Transforming an existing Reads: use the map method
2. Transforming an existing Writes: use contramap

However, contramap only works on Writes that produce JsObject. Your writes will fail at runtime:

val w = JsPath.write[String].contramap[B](_.a)
scala> w.writes(B("Hello"))
java.lang.RuntimeException: when empty JsPath, expecting JsObject

You can create a Writes "from scratch" using Writes.apply:

Writes[B](b => JsString(b.a))

Similarly you can create a Reads using Reads.apply.