General type parameter for nested lists

I'm trying to write a parser which should parse a Prolog list (for example [1,2,3,4]) into the corresponding Scala List. I programmed the parser with Scalas parsing combinators.

My parser looks like this so far:

class PListParser extends JavaTokenParsers{
    def list:Parser[List[Any]] = "[" ~> listArgs <~ "]"
    def listArgs:Parser[List[Any]] = list | repsep(args, "开发者_如何转开发,")
    def args:Parser[String] = "(.)*".r
}

Is there any possibility to turn the type parameters of the first two parsers into something more specific? Like a general parameter for nested lists of arbitrary dimension but the same underling type.

I think it should be trees, that is the proper structure for lists nested to arbitrary depth

sealed trait Tree[A]
case class Leaf[A](value: A) extends Tree[A] {
  override def toString: String = value.toString
}
case class Node[A](items: List[Tree[A]]) extends Tree[A] {
  override def toString: String = "Node(" + items.mkString(", ") + ")"
}

(Do the toString as you like, but I think the default one are rather too verbose)

Then, with minor fixes to your grammar (+ parse method, just to test easily on REPL)

object PrologListParser extends JavaTokenParsers{
    def list:Parser[Tree[String]] = "[" ~> listArgs <~ "]"
    def listArgs:Parser[Tree[String]] = repsep(list | args, ",") ^^ {Node(_)}
    def args:Parser[Tree[String]] = """([^,\[\]])*""".r ^^ {Leaf(_)}
    def parse(s: String): ParseResult[Tree[String]] = parse(list, s)
}


PrologListParser.parse("[a, b, [c, [d, e], f, [g], h], [i, j], k]")

res0: PrologList.ParseResult[Tree[String]] = [1.42] parsed: Node(a, b, Node(c, Node(d, e), f, Node(g), h), Node(i, j), k)

(Not tested)

sealed trait PrologTerm

case class PInt(i: Integer) extends PrologTerm { 
  override def toString = i.toString 
}
case class PAtom(s: String) extends PrologTerm { 
  override def toString = s.toString 
}
case class PComplex(f: PAtom, args: List[PrologTerm]) extends PrologTerm {
  override def toString = f.toString+"("+args.mkString(", ")+")"
}
case class PList(items: List[PrologTerm]) extends PrologTerm {
  override def toString = "["+items.mkString(", ")+"]"
}

object PrologListParser extends JavaTokenParsers {
  def term : Parser[PrologTerm] = int | complex | atom | list
  def int : Parser[PInt] = wholeNumber ^^ {s => PInt(s.toInt)}
  def complex : Parser[PComplex] = 
    (atom ~ ("(" ~> repsep(term, ",") <~ ")")) ^^ {case f ~ args => PAtom(f, args)}
  def atom : Parser[PAtom] = "[a-z][a-zA-Z_]*".r ^^ {PAtom(_)}
  def list : Parser[PList] = ("[" ~> repsep(term, ",") <~ "]") ^^ {PList(_)}
}