In Scala, how can I programmatically determine the name of the fields of a case class?

Question

In Scala, suppose I have a case class like this:

case class Sample(myInt: Int, myString: String)

Is there a way for me to obtain a Seq[(String, Class[_])], or better yet, Seq[(String, Manifest)], describing the case class's parameters?

Answer 1

It's me again (two years later). Here's a different, different solution using Scala reflection. It is inspired by a blog post, which was itself inspired by a Stack Overflow exchange. The solution below is specialized to the original poster's question above.

In one compilation unit (a REPL :paste or a compiled JAR), include scala-reflect as a dependency and compile the following (tested in Scala 2.11, might work in Scala 2.10):

import scala.language.experimental.macros 
import scala.reflect.macros.blackbox.Context

object CaseClassFieldsExtractor {
  implicit def makeExtractor[T]: CaseClassFieldsExtractor[T] =
    macro makeExtractorImpl[T]

  def makeExtractorImpl[T: c.WeakTypeTag](c: Context):
                              c.Expr[CaseClassFieldsExtractor[T]] = {
    import c.universe._
    val tpe = weakTypeOf[T]

    val fields = tpe.decls.collectFirst {
      case m: MethodSymbol if (m.isPrimaryConstructor) => m
    }.get.paramLists.head

    val extractParams = fields.map { field =>
      val name = field.asTerm.name
      val fieldName = name.decodedName.toString
      val NullaryMethodType(fieldType) = tpe.decl(name).typeSignature

      q"$fieldName -> ${fieldType.toString}"
    }

    c.Expr[CaseClassFieldsExtractor[T]](q"""
      new CaseClassFieldsExtractor[$tpe] {
        def get = Map(..$extractParams)
      }
    """)
  }
}

trait CaseClassFieldsExtractor[T] {
  def get: Map[String, String]
}

def caseClassFields[T : CaseClassFieldsExtractor] =
  implicitly[CaseClassFieldsExtractor[T]].get

And in another compilation unit (the next line in the REPL or code compiled with the previous as a dependency), use it like this:

scala> case class Something(x: Int, y: Double, z: String)
defined class Something

scala> caseClassFields[Something]
res0: Map[String,String] = Map(x -> Int, y -> Double, z -> String)

It seems like overkill, but I haven't been able to get it any shorter. Here's what it does:

1. The caseClassFields function creates an intermediate CaseClassFieldsExtractor that implicitly comes into existence, reports its findings, and disappears.
2. The CaseClassFieldsExtractor is a trait with a companion object that defines an anonymous concrete subclass of this trait, using a macro. It is the macro that can inspect your case class's fields because it has rich, compiler-level information about the case class.
3. The CaseClassFieldsExtractor and its companion object must be declared in a previous compilation unit to the one that examines your case class so that the macro exists at the time you want to use it.
4. Your case class's type data is passed in through the WeakTypeTag. This evaluates to a Scala structure with lots of pattern matching and no documentation that I could find.
5. We again assume that there's only one ("primary"?) constructor, but I think all classes defined in Scala can have only one constructor. Since this technique examines the fields of the constructor, not all JVM fields in the class, so it's not susceptible to the lack of generality that marred my previous solution.
6. It uses quasiquotes to build up an anonymous, concrete subclass of the CaseClassFieldsExtractor.
7. All that "implicit" business allows the macro to be defined and wrapped up in a function call (caseClassFields) without being called too early, when it's not yet defined.

Any comments that could refine this solution or explain how exactly the "implicits" do what they do (or if they can be removed) are welcome.