julienrf.variants.Variants.scala Maven / Gradle / Ivy
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package julienrf.variants
import scala.language.experimental.macros
import play.api.libs.json.{Writes, Reads, Format, __}
import scala.reflect.macros.Context
object Variants {
/**
* @tparam A The base type of a case class hierarchy.
* @return A [[play.api.libs.json.Format]] for the type hierarchy of `A`. It uses an additional field named `$variant`
* to discriminate between the possible subtypes of `A`.
*/
def format[A]: Format[A] = macro Impl.format[A]
/**
* @param discriminator Format of the type discriminator field.
* @tparam A Base type of case class hierarchy.
* @return A [[play.api.libs.json.Format]] for the type hierarchy of `A`.
*/
def format[A](discriminator: Format[String]): Format[A] = macro Impl.formatDiscriminator[A]
/**
* @tparam A The base type of a case class hierarchy.
* @return A [[play.api.libs.json.Reads]] for the type hierarchy of `A`. It relies on an additional field named `$variant`
* to discriminate between the possible subtypes of `A`.
*/
def reads[A]: Reads[A] = macro Impl.reads[A]
/**
* @param discriminator Decoder of the type discriminator field.
* @tparam A Base type of case class hierarchy.
* @return A [[play.api.libs.json.Reads]] for the type hierarchy of `A`.
*/
def reads[A](discriminator: Reads[String]): Reads[A] = macro Impl.readsDiscriminator[A]
/**
* @tparam A The base type of a case class hierarchy.
* @return A [[play.api.libs.json.Writes]] for the type hierarchy of `A`. It uses an additional field named `$variant`
* to discriminate between the possible subtypes of `A`.
*/
def writes[A]: Writes[A] = macro Impl.writes[A]
/**
* @param discriminator Name of the type discriminator field.
* @tparam A Base type of case class hierarchy.
* @return A [[play.api.libs.json.Writes]] for the type hierarchy of `A`.
*/
def writes[A](discriminator: Writes[String]): Writes[A] = macro Impl.writesDiscriminator[A]
private object Impl {
val defaultDiscriminator = (__ \ "$variant").format[String]
/**
* Given the following definition of class hierarchy `Foo`:
*
* {{{
* sealed trait Foo
* case class Bar(x: Int) extends Foo
* case class Baz(s: String) extends Foo
* case object Bah extends Foo
* }}}
*
* `Variants.format[Foo]` expands to the following:
*
* {{{
* {
* import play.api.libs.json.{Writes, Reads}
*
* val writes = Writes[Foo] {
* case bar: Bar => Json.toJson(bar)(Json.writes[Bar]).as[JsObject] + ("$variant" -> JsString("Bar"))
* case baz: Baz => Json.toJson(baz)(Json.writes[Baz]).as[JsObject] + ("$variant" -> JsString("Baz"))
* case _: Bah => JsObject(Seq("$variant" -> JsString("Bah")))
* }
*
* val reads = Reads[Foo] { json =>
* (json \ "$variant").validate[String].flatMap {
* case "Bar" => Json.fromJson(json)(Json.reads[Bar])
* case "Baz" => Json.fromJson(json)(Json.reads[Baz])
* case "Bah" => JsSuccess(Bah)
* }
* }
*
* Format(reads, writes)
* }
*
* }}}
*
*/
def format[A : c.WeakTypeTag](c: Context): c.Expr[Format[A]] = {
import c.universe._
formatDiscriminator[A](c)(reify(defaultDiscriminator))
}
def formatDiscriminator[A : c.WeakTypeTag](c: Context)(discriminator: c.Expr[Format[String]]): c.Expr[Format[A]] = {
import c.universe._
val (baseClass, variants) = baseAndVariants[A](c)
val writes = writesTree(c)(baseClass, variants, discriminator)
val reads = readsTree(c)(baseClass, variants, discriminator)
c.Expr[Format[A]](q"play.api.libs.json.Format[$baseClass]($reads, $writes)")
}
def reads[A : c.WeakTypeTag](c: Context): c.Expr[Reads[A]] = {
import c.universe._
readsDiscriminator[A](c)(reify(defaultDiscriminator))
}
def readsDiscriminator[A : c.WeakTypeTag](c: Context)
(discriminator: c.Expr[Reads[String]]): c.Expr[Reads[A]] = {
import c.universe._
val (baseClass, variants) = baseAndVariants[A](c)
c.Expr[Reads[A]](readsTree(c)(baseClass, variants, discriminator))
}
def writes[A : c.WeakTypeTag](c: Context): c.Expr[Writes[A]] = {
import c.universe._
writesDiscriminator[A](c)(reify(defaultDiscriminator))
}
def writesDiscriminator[A : c.WeakTypeTag](c: Context)(discriminator: c.Expr[Writes[String]]): c.Expr[Writes[A]] = {
val (baseClass, variants) = baseAndVariants[A](c)
c.Expr[Writes[A]](writesTree(c)(baseClass, variants, discriminator))
}
/*
* Get the class hierarchy and checks that the hierarchy is closed
*/
def baseAndVariants[A : c.WeakTypeTag](c: Context): (c.universe.ClassSymbol, Set[c.universe.ClassSymbol]) = {
import c.universe._
val baseClass = weakTypeOf[A].typeSymbol.asClass
baseClass.typeSignature // SI-7046
if (!baseClass.isSealed) {
c.abort(c.enclosingPosition, s"$baseClass is not sealed")
}
// Get all the possible variants of this type
val variants = baseClass.knownDirectSubclasses.map(_.asClass)
for (variant <- variants if !(variant.isCaseClass || variant.isModuleClass)) {
c.abort(c.enclosingPosition, s"$variant is not a case class nor a case object")
}
baseClass -> variants
}
def writesTree(c: Context)(baseClass: c.universe.ClassSymbol, variants: Set[c.universe.ClassSymbol], discriminator: c.Expr[Writes[String]]): c.Tree = {
import c.universe._
val writesCases = for (variant <- variants) yield {
if (!variant.isModuleClass) {
val term = newTermName(c.fresh())
cq"""$term: $variant => play.api.libs.json.Json.toJson($term)(play.api.libs.json.Json.writes[$variant]).as[play.api.libs.json.JsObject] ++ $discriminator.writes(${variant.name.decodedName.toString})"""
} else {
cq"""_: $variant => $discriminator.writes(${variant.name.decodedName.toString})"""
}
}
q"play.api.libs.json.Writes[$baseClass] { case ..$writesCases }"
}
def readsTree(c: Context)(baseClass: c.universe.ClassSymbol, variants: Set[c.universe.ClassSymbol], discriminator: c.Expr[Reads[String]]): c.Tree = {
import c.universe._
val readsCases = for (variant <- variants) yield {
if (!variant.isModuleClass) {
cq"""${variant.name.decodedName.toString} => play.api.libs.json.Json.fromJson(json)(play.api.libs.json.Json.reads[$variant])"""
} else {
cq"""${variant.name.decodedName.toString} => play.api.libs.json.JsSuccess(${newTermName(variant.name.decodedName.toString)})"""
}
}
q"""
play.api.libs.json.Reads[$baseClass](json =>
$discriminator.reads(json).flatMap { case ..$readsCases }
)
"""
}
}
}
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