我想编写一个 scala 宏，它可以基于带有简单类型检查的映射条目覆盖案例类的字段值。如果原始字段类型和覆盖值类型兼容，则设置新值，否则保留原始值。
到目前为止，我有以下代码：
import language.experimental.macros
import scala.reflect.macros.Context
object ProductUtils {
def withOverrides[T](entity: T, overrides: Map[String, Any]): T =
macro withOverridesImpl[T]
def withOverridesImpl[T: c.WeakTypeTag](c: Context)
(entity: c.Expr[T], overrides: c.Expr[Map[String, Any]]): c.Expr[T] = {
import c.universe._
val originalEntityTree = reify(entity.splice).tree
val originalEntityCopy = entity.actualType.member(newTermName("copy"))
val originalEntity =
weakTypeOf[T].declarations.collect {
case m: MethodSymbol if m.isCaseAccessor =>
(m.name, c.Expr[T](Select(originalEntityTree, m.name)), m.returnType)
}
val values =
originalEntity.map {
case (name, value, ctype) =>
AssignOrNamedArg(
Ident(name),
{
def reifyWithType[K: WeakTypeTag] = reify {
overrides
.splice
.asInstanceOf[Map[String, Any]]
.get(c.literal(name.decoded).splice) match {
case Some(newValue : K) => newValue
case _ => value.splice
}
}
reifyWithType(c.WeakTypeTag(ctype)).tree
}
)
}.toList
originalEntityCopy match {
case s: MethodSymbol =>
c.Expr[T](
Apply(Select(originalEntityTree, originalEntityCopy), values))
case _ => c.abort(c.enclosingPosition, "No eligible copy method!")
}
}
}
像这样执行：
import macros.ProductUtils
case class Example(field1: String, field2: Int, filed3: String)
object MacrosTest {
def main(args: Array[String]) {
val overrides = Map("field1" -> "new value", "field2" -> "wrong type")
println(ProductUtils.withOverrides(Example("", 0, ""), overrides)) // Example("new value", 0, "")
}
}
正如你所看到的，我已经设法获得了原始字段的类型，现在想在reifyWithType.
不幸的是，在当前的实现中，我在编译期间收到了警告：
warning: abstract type pattern K is unchecked since it is eliminated by erasure case Some(newValue : K) => newValue
和 IntelliJ 中的编译器崩溃：
Exception in thread "main" java.lang.NullPointerException
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.preEraseAsInstanceOf$1(Erasure.scala:1032)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.preEraseNormalApply(Erasure.scala:1083)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.preEraseApply(Erasure.scala:1187)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.preErase(Erasure.scala:1193)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.transform(Erasure.scala:1268)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.transform(Erasure.scala:1018)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1217)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:13)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:13)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2897)
at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.transform(TypingTransformers.scala:48)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.transform(Erasure.scala:1280)
at scala.tools.nsc.transform.Erasure$ErasureTransformer$$anon$1.transform(Erasure.scala:1018)
所以问题是：
* 是否可以将宏中接收的类型与值运行时类型进行类型比较？
* 或者有没有更好的方法来解决这个任务？