output.txt

package foo {
  import bar._;
  object HelloWorld extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    def main(args: Array[String]): scala.Unit = println("Hello, world!")
  }
}
----
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  Ident
  Import
    Ident
  ModuleDef
    Template
      Select
        Ident
      emptyValDef$
        TypeTree
      DefDef
        TypeTree
        Block
          Apply
            Select
              Super
                This
          Literal
      DefDef
        ValDef
          AppliedTypeTree
            Ident
            Ident
        Select
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          Ident
          Literal

ScalaParser.scala

import scala.tools.nsc.{ast, Settings}
import scala.tools.nsc.interpreter._
import scala.tools.nsc.util.BatchSourceFile

class ScalaParser {
  val settings = new Settings()
  settings.usejavacp.value = true
  val main = new IMain(settings)

  import main.global._
  import syntaxAnalyzer._

  def parse(code: String) = {
    val parser = new SourceFileParser(new BatchSourceFile("<console>", code))
    parser.compilationUnit()
  }

  object traverser {
    def traverse(trees: List[Tree], level: Int)(printTree: (Tree, Int) => Unit): Unit = {
      for (tree <- trees) {
        traverse(tree, level){printTree}
      }
    }

    def traverse(tree: Tree, level: Int)(printTree: (Tree, Int) => Unit): Unit = {
      printTree(tree, level)
      traverse(tree.children, level + 1){printTree}
    }
  }

  class TreePrinter {
    def print(trees: List[Tree]): Unit = {
      def printTree(tree: Tree, nest: Int) {
        val indent = "  " * nest
        println(indent + tree.getClass.getSimpleName())
      }
      traverser.traverse(trees, 0){printTree}
    }

    def print(tree: Tree): Unit = print(List(tree))
  }
}

ScalaParserMain.scala

object ScalaParserMain {
  def main(args: Array[String]) {
    // TODO: deal a file via args
    val code =
      """
        |package foo
        |import bar._
        |object HelloWorld {
        |  def main(args: Array[String]) {
        |    println("Hello, world!")
        |  }
        |}
      """.stripMargin
    val parser = new ScalaParser()
    val tree = parser.parse(code)

    val printer = new parser.TreePrinter()
    println(tree)
    println("----")
    printer.print(tree)
  }
}