Toy language with parser combinators of scala, which is derived from Kishida-san's code. Original language has dynamic scope. But It has static scope and so-called lexical closure.

MiniParser.scala

package miniparser

import scala.util.parsing.combinator.RegexParsers

object Main {

  def main(args: Array[String]): Unit = {
    val parser = new MiniParser
    println(parser.parse("12-5*3+7*(2+5)+0"))
  }

  trait AST
  case class AddOp(left: AST, right:AST) extends AST
  case class SubOp(left: AST, right:AST) extends AST
  case class MulOp(left: AST, right:AST) extends AST
  case class IntVal(value: Int) extends AST
  
  class MiniParser extends RegexParsers{
    //expr ::= add
    def expr: Parser[AST] = add
    //expr ::= term {"+" term | "-" term}.
    def add:  Parser[AST] = term~rep("+"~term|"-"~term)^^{
      case left~rest => {
          var ast = left
          rest.foreach{
            case "+"~right => {ast = AddOp(ast, right)}
            case "-"~right => {ast = SubOp(ast, right)}
          }
          ast
      }
    }
    //term ::= factor {"*" factor}
    def term : Parser[AST] = factor~rep("*"~factor)^^{
      case left~rest => {
          var ast = left
          rest.foreach{
            case _~right => {ast = MulOp(ast, right)}
          }
          ast
      }
    }
    //factor ::= intLiteral | "(" expr ")"
    def factor: Parser[AST] = intLiteral | "("~expr~")"^^{
      case _~exp~_ => exp
    }
    //intLiteral ::= ["1"-"9"] {"0"-"9"}
    def intLiteral : Parser[AST] = """[1-9][0-9]*|0""".r^^{
      case value => IntVal(value.toInt)
    }
    
    def parse(str:String) = parseAll(expr, str)
  }
}