Scala：避免类型擦除

Question

From the following code:

type Structure = Map[String, Any]

def getStructureSourceNames(structuresDesc: Structure): Iterable[String] = {
  val subColsDesc: Map[String, String] =
    structuresDesc.filter(_._2.isInstanceOf[String]).asInstanceOf[Map[String, String]]
  val subStructuresDesc: Map[String, Structure] = structuresDesc
    .filter(_._2.isInstanceOf[Map[String, Structure]])
    .asInstanceOf[Map[String, Structure]]

  subColsDesc.values ++ subStructuresDesc.values.flatMap(getStructureSourceNames(_))
}

I want to pass a recursive Map of (String -> String), ia. an example of Structure is:

Map("test" -> Map(
        "newid" -> "id",
        "newstring" -> "string",
        "toto" -> Map("newdouble" -> "double")
      ),
      "otherid" -> "id")

The method getStructureSourceNames should return the list of "final" value, ia. browse the whole tree and for each leaft, get the String value.

When I run this code, this drives me to:

Warning:(78, 32) non-variable type argument String in type scala.collection.immutable.Map[String,Structure] (the underlying of Map[String,Structure]) is unchecked since it is eliminated by erasure
  .filter(_._2.isInstanceOf[Map[String, Structure]])

Moreover, I don't like to use isInstanceOf / asInstanceOf. By googling, I found that I could use pattern matching to check for the type, and get the Map with expected typing, but I can"t find how to do it.

Would you have an example of such code?

Answer 1

There are 2 kinds of pattern matching:

1) pattern matching on a sealed trait (good)

2) pattern matching where patterns involve matching with arbitrary classes and equality checks (not better than instanceOf checks)

To avoid 2) you need to make the type you want to match a sealed trait:

sealed trait ConfigValue
case class StringValue(v: String) extends ConfigValue
case class MapValue(map: Map[String, ConfigValue]) extends ConfigValue

val struct: ConfigValue = MapValue(Map("key1" -> StringValue("v1"),
                   "key2" -> MapValue(Map("sub" -> StringValue("val")))))

def allValues(s: ConfigValue): Iterable[String] = {
  s match {
    case StringValue(v) => Seq(v)
    case MapValue(map) => map.values.flatMap(v => allValues(v))
  }
}

println(allValues(struct))

By the way your structure looks similar to json. Maybe you could reuse some json library.