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@rjsen /Extends.scala
Last active Mar 28, 2016

What would you like to do?
Macro annotation to eliminate case class repetition
import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context
@compileTimeOnly("use macro paradise")
class Extends[T](defaults: Any*) extends StaticAnnotation {
def macroTransform(annottees: Any*): Any = macro Extends.impl
}
object Extends {
def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
import c.universe._
val targetTrait: Tree = c.prefix.tree match {
case q"new $name[$typ]" => typ
}
val tpe: Type = c.typecheck(q"0.asInstanceOf[$targetTrait]").tpe
val values: List[TermSymbol] = tpe.members.collect { case d if d.isTerm => d.asTerm }.filter(t => t.isStable && t.isMethod && t.isAbstract).toList.reverse
def extractClassNameAndFields(classDecl: ClassDef): (TypeName, Seq[Tree], Seq[Tree], Seq[Tree]) = {
try {
val q"case class $className(..$fields) extends ..$bases { ..$body }" = classDecl
(className, fields, bases, body)
} catch {
case _: MatchError => c.abort(c.enclosingPosition, "Annotation is only supported on case classes")
}
}
def modifiedFields(fields: List[ValDef]): List[ValDef] = {
val newParams = values.map { v =>
ValDef(Modifiers(Flag.CASEACCESSOR), v.name, TypeTree(v.typeSignature.typeConstructor), EmptyTree)
}
newParams ++ fields
}
def modifiedClass(classDecl: ClassDef): Expr[_] = {
val (className, fields, bases, body) = extractClassNameAndFields(classDecl)
val ctor = modifiedFields(fields.asInstanceOf[List[ValDef]])
c.Expr(q"case class $className(..$ctor) extends $targetTrait with ..$bases { ..$body }")
}
annottees.map(_.tree) match {
case (classDecl: ClassDef) :: Nil => modifiedClass(classDecl)
case _ => c.abort(c.enclosingPosition, "Invalid annottee")
}
}
}
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