8÷2(3+1) に対する二通りのパーザ（あるいはインタプリタ）作ってみた

Calculator1.scala

import com.github.kmizu.scomb._

object Calculator1 extends SCombinator[Int] {
  def root: Parser[Int] = expression

  def expression: Parser[Int] = rule(A)

  def A: Parser[Int] = rule(chainl(M) {
    $("+").map { op => (lhs: Int, rhs: Int) => lhs + rhs } |
    $("-").map { op => (lhs: Int, rhs: Int) => lhs - rhs }
  })

  def M: Parser[Int] = rule(
    chainl(N) {
      $("×").map { op => (lhs: Int, rhs: Int) => lhs * rhs } |
      $("÷").map { op => (lhs: Int, rhs: Int) => lhs / rhs }
    }
  )

  def N: Parser[Int] = rule {
    (P ~ P.*).map{ case a ~ bs => bs.foldLeft(a){_ * _} }
  }

  def P: P[Int] = rule{ K | number }

  def K: P[Int] =
    (for {
        _ <- string("("); e <- expression; _ <- string(")")} yield e)

  def number: P[Int] = rule(set('0'to'9').+.map{ digits => digits.mkString.toInt})
}

Calculator2.scala

import com.github.kmizu.scomb._

object Calculator2 extends SCombinator[Int] {
  def root: Parser[Int] = expression

  def expression: Parser[Int] = rule(A)

  def A: Parser[Int] = rule(chainl(N) {
    $("+").map { op => (lhs: Int, rhs: Int) => lhs + rhs } |
    $("-").map { op => (lhs: Int, rhs: Int) => lhs - rhs }
  })

  def N: Parser[Int] = rule {
    (M ~ M.*).map{ case a ~ bs => bs.foldLeft(a){_ * _} }
  }

  def M: Parser[Int] = rule(
    chainl(P) {
      $("×").map { op => (lhs: Int, rhs: Int) => lhs * rhs } |
      $("÷").map { op => (lhs: Int, rhs: Int) => lhs / rhs }
    }
  )


  def P: P[Int] = rule{ K | number }

  def K: P[Int] =
    (for {
        _ <- string("("); e <- expression; _ <- string(")")} yield e)

  def number: P[Int] = rule(set('0'to'9').+.map{ digits => digits.mkString.toInt})
}

console.txt

[mizushima]$ sbt console
warning: ignoring extraneous `sbt-` prefix in version `sbt-1.5.2`
         (set by /home/mizushima/work/implicit_multiplication/project/build.properties)
[info] welcome to sbt 1.5.2 (Ubuntu Java 11.0.9.1)
[info] loading project definition from /home/mizushima/work/implicit_multiplication/project
[info] loading settings for project implicit_multiplication from build.sbt ...
[info] set current project to implicit_multiplication (in build file:/home/mizushima/work/implicit_multiplication/)
[info] Starting scala interpreter...
Welcome to Scala 2.13.4 (OpenJDK 64-Bit Server VM, Java 11.0.9.1).
Type in expressions for evaluation. Or try :help.

scala> Calculator1.parse("8÷2(3+1)")
val res0: com.github.kmizu.scomb.Result[Int] = Success(1) // 暗黙の乗算を優先した場合

scala> Calculator2.parse("8÷2(3+1)")
val res1: com.github.kmizu.scomb.Result[Int] = Success(16) // 除算を優先した場合