how to get value class fields with scala reflection

[Jeff Olson]

I have some class with a value class field. I would like to use scala reflection to get at that field, but so far I can seem to do no better than I can do with java reflection. Here's an example:

mport scala.reflect.runtime._

case class Foo(x: Int) extends AnyVal

case class Bar(foo: Foo)

object Test {

  def main(args: Array[String]) {

    val bar = Bar(Foo(3))

    println(bar.foo) // Foo(3)

    val mirror = currentMirror

    val im = mirror.reflect(bar)

    val fooTerm = universe.typeOf[Bar].declaration(universe.newTermName("foo")).asMethod

    val obj1 = im.reflectField(fooTerm).get // returns java.lang.Integer(3) not Foo(3)

    println(obj1) // 3

    val obj2 = im.reflectMethod(fooTerm)() // returns java.lang.Integer(3) not Foo(3)

    println(obj2) // 3

  }

}

So how do I get Foo(3) from my InstanceMirror `im` and my MethodSymbol `fooTerm`? Obviously I would like a solution with works with arbitrary value class fields, not just this silly example.

-Jeff

[Flavio W. Brasil]

Jeff,

It looks like that when using runtime reflection, foo becomes the int value (3) because it extends AnyVal.

Eugene,

This behavior is expected or are we using the reflection API in a wrong way? I get the same result using SMirror:

scala>  implicit val mirror = scala.reflect.runtime.currentMirror

mirror: reflect.runtime.universe.Mirror = JavaMirror with scala.t...

scala> import net.fwbrasil.smirror._

import net.fwbrasil.smirror._

scala> case class Foo(x: Int) extends AnyVal

defined class Foo

scala> case class Bar(foo: Foo)

defined class Bar

scala> val bar = Bar(Foo(3))

bar: Bar = Bar(Foo(3))

scala> println(bar.foo)

Foo(3)

scala> bar.reflect.vals

res3: List[net.fwbrasil.smirror.SInstanceVal[Bar]] = List(val foo: Foo (bound to Bar(Foo(3))))

scala> bar.reflect.vals.head.get

res4: Any = 3

-- 

Flávio W. Brasil

http://activate-framework.org

{persistence as it should be}

--
You received this message because you are subscribed to the Google Groups "scala-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email to scala-user+...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.
 
 

[Jeff Olson]

Yes, but it knows that the field is of type Foo, so, in theory, it should be capable of doing the right thing:

scala> println(fooTerm.returnType)

res10: Foo

-Jeff

[Eugene Burmako]

Could you please submit an issue for this one? If that's a blocker for you, I could fix it soonish.

[Jeff Olson]

Here you go: https://issues.scala-lang.org/browse/SI-7328

It is a blocker, but I can't wait for a fix, so I'll have to cook up a workaround.

-Jeff

[Jeff Olson]

so here's my cooked up workaround if anyone else runs into this before Eugene can fix it:

  def getFieldValue(im: InstanceMirror, term: MethodSymbol): Any = {

    val rawValue = im.reflectMethod(term)()

    val tpe = term.returnType

    val symbol = tpe.typeSymbol.asClass

    if (symbol.isDerivedValueClass) {

      val ctor = tpe.declaration(nme.CONSTRUCTOR).asMethod

      val ctorMirror = mirror.reflectClass(symbol).reflectConstructor(ctor)

      ctorMirror(rawValue)

    }

    else rawValue

  }

-Jeff