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 expr = "12-5*3+7*(2+5)*1+0"
    
    val parser = new MiniParser
    val ast = parser.parse(expr).get

    val visitor = new ExprVisitor
    var result = visitor.visit(ast);
    
    println(result)
  }
}

trait Visitor{
  def visit(ast:AST):Any
}

class ExprVisitor extends Visitor{
  def visit(ast:AST):Any = {
    ast match{
      case AddOp(left, right) =>{
          (left.accept(this), right.accept(this)) match{
            case (lval:Int, rval:Int) => lval + rval
          }
      }
      case SubOp(left, right) =>{
          (left.accept(this), right.accept(this)) match{
            case (lval:Int, rval:Int) => lval - rval
          }
      }
      case MulOp(left, right) =>{
          (left.accept(this), right.accept(this)) match{
            case (lval:Int, rval:Int) => lval * rval
          }
      }
      case IntVal(value) => value
    }
  }
}

trait AST{
  def accept(visitor:Visitor) = {
    visitor.visit(this)
  }
}

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] = chainl1(term,
    "+"^^{case op => (left:AST, right:AST) => AddOp(left, right)}|
    "-"^^{case op => (left:AST, right:AST) => SubOp(left, right)})
  //term ::= factor {"*" factor}
  def term : Parser[AST] = chainl1(factor, 
    "*"^^{case op => (left:AST, right:AST) => MulOp(left, right)})
  //factor ::= intLiteral | "(" expr ")"
  def factor: Parser[AST] = intLiteral | "("~>expr<~")"^^(
    x=>x)
  //intLiteral ::= ["1"-"9"] {"0"-"9"}
  def intLiteral : Parser[AST] = """[1-9][0-9]*|0""".r^^(
    value => IntVal(value.toInt))

  def parse(str:String) = parseAll(expr, str)
}