add Defaults giving access to a class's Default values

Author: cvogt

readme.txt

@@ -5,6 +5,7 @@ Contents:
 
 Type-level helpers
 - EnumerateSingletons - listing all objects extending a sealed trait
+- Defaults - object giving names access to a class's default values
 
 Collection extensions (org.cvogt.collection)
 - distinctBy - remove duplicates by key

src/main/scala/Defaults.scala

@@ -0,0 +1,44 @@
+package org.cvogt.scala
+
+import scala.reflect.macros.whitebox.Context
+import scala.language.dynamics
+import scala.language.experimental.macros
+import macrocompat.bundle
+
+object Defaults {
+ /** returns the given type's default values in an anonymous class instance */
+ def apply[T]: Any = macro DefaultsMacros.apply[T]
+}
+
+@bundle
+class DefaultsMacros( val c: Context ) {
+ import c.universe._
+ def apply[T: c.WeakTypeTag] = {
+ val T = weakTypeOf[T]
+ if ( !isCaseClass( T ) )
+ c.error( c.enclosingPosition, s"not a case class: $T" )
+ val defs = caseClassFieldsTypes( T ).collect {
+ case ( name, tpe, Some( default ) ) => q"def ${TermName( name )}: $tpe = $default"
+ }
+ q"new{ ..$defs }"
+ }
+
+ private def isCaseClass( tpe: Type ) = tpe.typeSymbol.isClass && tpe.typeSymbol.asClass.isCaseClass
+
+ private def caseClassFieldsTypes( tpe: Type ): List[( String, Type, Option[Tree] )] = {
+ tpe.companion.member( TermName( "apply" ) ).asTerm.alternatives.find( _.isSynthetic ).get.asMethod.paramLists.flatten.zipWithIndex.map {
+ case ( field, i ) =>
+ (
+ field.name.toTermName.decodedName.toString,
+ field.infoIn( tpe ),
+ {
+ val method = TermName( s"apply$$default$$${i + 1}" )
+ tpe.companion.member( method ) match {
+ case NoSymbol => None
+ case _ => Some( q"${tpe.typeSymbol.companion}.$method" )
+ }
+ }
+ )
+ }.toList
+ }
+}

src/test/scala/DefaultsTest.scala

@@ -0,0 +1,15 @@
+package org.cvogt.scala.test
+
+import org.cvogt.scala.Defaults
+
+import org.scalatest.FunSuite
+
+case class Foo(i: Int = 5, s: String)
+class DefaultsTest extends FunSuite {
+ test( "case class" ) {
+ val d = Defaults[Foo]
+
+ import scala.language.reflectiveCalls
+ assert( d.i === 5 )
+ }
+}