Defining `Reads` for JSON Set Type

问题:

How can I create a play.api.libs.Reads for my People case class?

scala> type Id = Long
defined type alias Id

scala> case class People(names: Set[Id])
defined class People

scala>   implicit val PeopleReads: Reads[People] = (
     |     (__ \ "names").read[Set[Id]])(People)
<console>:21: error: overloaded method value read with alternatives:
  (t: Set[Id])play.api.libs.json.Reads[Set[Id]] <and>
  (implicit r: play.api.libs.json.Reads[Set[Id]])play.api.libs.json.Reads[Set[Id]]
 cannot be applied to (People.type)
           (__ \ "names").read[Set[Id]])(People)

回答1:

The (...)(People) syntax is designed for when you've built up a list of arguments (well, technically it's a Builder, not a list) with and and want to lift the People constructor into the applicative functor for Reads so that you can apply it to those arguments.

For example, if your People type looked like this:

case class People(names: Set[Id], home: String)

You could write:

implicit val PeopleReads: Reads[People] = (
  (__ \ "names").read[Set[Id]] and
  (__ \ "home").read[String]
)(People)

In your case, though, the constructor for People has a single argument, and you haven't used and, so you don't have a Builder[Reads[Set[Id] ~ String], you've just got a plain old Reads[Set[Id]].

This is nice, because it means you don't need the weird applicative functor syntax—all you need is map:

implicit val PeopleReads = (__ \ "names").read[Set[Id]].map(People)

And you're done.