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

I'm answering my own question to provide a base solution, but I'm looking for alternatives and improvements, too.

---

One option, also compatible with Java and not restricted to case classes, is to use ParaNamer. In Scala, another option is to parse the ScalaSig bytes attached to generated classfiles. Both solutions won't work in the REPL.

Here's my attempt at extracting the names of the fields from ScalaSig (which uses scalap and Scala 2.8.1):

def valNames[C: ClassManifest]: Seq[(String, Class[_])] = {
  val cls = classManifest[C].erasure
  val ctors = cls.getConstructors

  assert(ctors.size == 1, "Class " + cls.getName + " should have only one constructor")
  val sig = ScalaSigParser.parse(cls).getOrElse(error("No ScalaSig for class " + cls.getName + ", make sure it is a top-level case class"))

  val classSymbol = sig.parseEntry(0).asInstanceOf[ClassSymbol]
  assert(classSymbol.isCase, "Class " + cls.getName + " is not a case class")

  val tableSize = sig.table.size
  val ctorIndex = (1 until tableSize).find { i =>
    sig.parseEntry(i) match {
      case m @ MethodSymbol(SymbolInfo("<init>", owner, _, _, _, _), _) => owner match {
        case sym: SymbolInfoSymbol if sym.index == 0 => true
        case _ => false
      }
      case _ => false
    }
  }.getOrElse(error("Cannot find constructor entry in ScalaSig for class " + cls.getName))

  val paramsListBuilder = List.newBuilder[String]
  for (i <- (ctorIndex + 1) until tableSize) {
    sig.parseEntry(i) match {
      case MethodSymbol(SymbolInfo(name, owner, _, _, _, _), _) => owner match {
        case sym: SymbolInfoSymbol if sym.index == ctorIndex => paramsListBuilder += name
        case _ =>
      }
      case _ =>
    }
  }

  paramsListBuilder.result zip ctors(0).getParameterTypes
}

Disclaimer: I don't really understand the structure of ScalaSig and this should be considered as a heuristics. In particular, this code makes the following assumptions:

• Case classes have only one constructor.
• The entry of the signature at position zero is always a ClassSymbol.
• The relevant constructor of the class is the first MethodEntry with name <init> whose owner has id 0.
• The parameter names have as owner the constructor entry and always after that entry.

It will fail (because of no ScalaSig) on nested case classes.

This method also only returns Class instances and not Manifests.

Please feel free to suggest improvements!

Answer 2

Here's a different solution that uses plain-Java reflection.

case class Test(unknown1: String, unknown2: Int)
val test = Test("one", 2)

val names = test.getClass.getDeclaredFields.map(_.getName)
// In this example, returns Array(unknown1, unknown2).

To get a Seq[(String, Class[_])], you can do this:

val typeMap = test.getClass.getDeclaredMethods.map({
                x => (x.getName, x.getReturnType)
              }).toMap[String, Class[_]]
val pairs = names.map(x => (x, typeMap(x)))
// In this example, returns Array((unknown1,class java.lang.String), (two,int))

I'm not sure about how to get Manifests.