Play JSON Reads/Writes with single-parameter case classes

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.