Scala IF-ELSEIF-ELSE Simple parser combinator for control flow

Question

I am writing a parser combinator to parse simple control flow statements and execute some code. The structure of language is roughly this -

val resultId = 200
  val s = s"""(IF $resultId == 100 GOTO NODE-1-->NODE-2) (ELSE IF $resultId > 100 GOTO NODE-1-->NODE-3) (ELSE GOTO NODE-1-->NODE-4)""".stripMargin
  private val result= new ConditionalParserCombinator().run(s)

In above scenario for example I should get GOTO NODE-1-->NODE-3 instead I get false after evaluation of else expression, code of combinator outlined below:

final class ConditionalParserCombinator extends JavaTokenParsers with ParserCombinatorLike {
    def IF = "IF"
    def ELSE = "ELSE"
    def ELSEIF = ELSE ~ IF
    def NULL = "NULL"
    def GOTO = "GOTO"
    def node_id = wholeNumber | floatingPointNumber | stringLiteral
    def NODE = "NODE" ~ "-" ~ node_id ^^ (e ⇒ NodeExpression(e._2))
    def EDGE = NODE ~ "-->" ~ NODE ^^ (e ⇒ EdgeExpression(e._1._1, e._2))
    def lhs = ident | wholeNumber | floatingPointNumber | stringLiteral
    def rhs = ident | wholeNumber | floatingPointNumber | stringLiteral | NULL
    def operator = "==" | "*" | "/" | "||" | "&&" | ">" | "<" | ">=" | "<="
    def block = GOTO ~ EDGE
    def expression_block = lhs ~ operator ~ rhs ~ block ^^ {
      case lhs ~ operator ~ rhs ~ block ⇒ ExpressionBlock(lhs, rhs, operator, block._2)
    }
    def ifExpression = IF ~ expression_block ^^ (e ⇒ e._2.operator match {
      case "==" ⇒ if (e._2.lhs == e._2.rhs) Block(e._2.block) else false
      case ">" ⇒ if (e._2.lhs > e._2.rhs) Block(e._2.block) else false
      case "<" ⇒ if (e._2.lhs < e._2.rhs) Block(e._2.block) else false
      case _ ⇒ false
    })
    def elseIFExpression = ELSEIF ~ expression_block ^^ (e ⇒ e._2.operator match {
      case "==" ⇒ if (e._2.lhs == e._2.rhs) Block(e._2.block) else false
      case ">" ⇒ if (e._2.lhs > e._2.rhs) {
        println("matched elseif")
        Block(e._2.block)
      } else false
      case "<" ⇒ if (e._2.lhs < e._2.rhs) Block(e._2.block) else false
      case _ ⇒ false
    })
    def elseExpression = ELSE ~ block ^^ (e ⇒ Block(e._2._2))
    override def grammar = "(" ~> log(ifExpression)("ifexpression") <~ ")" ~!
      "(" ~> log(elseIFExpression)("elseifexpression") <~ ")" ~!
      "(" ~> log(elseExpression)("elseexpression") <~ ")"
  }

I am printing result.get and I see false as the result.

** Additional details - Block, ExpressionBlock are all case classes useful for a few things that I may do later on**

anirudh.vyas · Accepted Answer · 2018-06-25 23:55:43Z

I think its cleaner to parse an expression to a type that you can understand (meaning I have custom Product/Case classes defined for it) and then Evaluate it - these are two different things. In hindsight not sure why I got both mixed up. Here's the logic that works -

def IF = "IF"
def ELSE = "ELSE"
def ELSEIF = ELSE ~ IF
def NULL = "NULL"
def GOTO = "GOTO"
def dataType: Parser[DataType] = "[" ~ "Integer" ~ "]" ^^ { e ⇒ DataType("", "Integer") }
def node_id = wholeNumber | floatingPointNumber | stringLiteral
def NODE = "NODE" ~ "-" ~ node_id ^^ (e ⇒ ParseableNode(e._2, DataType({}, "Unit")))
def EDGE = NODE ~ "-->" ~ NODE ^^ (e ⇒ EdgeExpression(e._1._1, e._2))
def lhs = ident | wholeNumber | floatingPointNumber | stringLiteral
def rhs = ident | wholeNumber | floatingPointNumber | stringLiteral | NULL
def operator = "==" | "*" | "/" | "||" | "&&" | ">" | "<" | ">=" | "<="
def block = GOTO ~ EDGE
def expression_block(expType: ConditionalKind) = dataType ~ lhs ~ operator ~ rhs ~ block ^^ {
  case dataType ~ lhs ~ operator ~ rhs ~ block ⇒ ExpressionBlock(ParseableNode(lhs, dataType), ParseableNode(rhs, dataType), operator, block._2, expType)
}
def ifExpression = IF ~ expression_block(ConditionalKind("IF")) ^^ {
  case "IF" ~ expression_block ⇒ ExpressionBlock(expression_block.lhs, expression_block.rhs, expression_block.operator, expression_block.block, expression_block.conditionalKind)
}
def elseIFExpression = ELSEIF ~ expression_block(ConditionalKind("ELSEIF")) ^^ {
  case "ELSE" ~ "IF" ~ expression_block ⇒ ExpressionBlock(expression_block.lhs, expression_block.rhs, expression_block.operator, expression_block.block, expression_block.conditionalKind)
}
def elseExpression = ELSE ~ block ^^ { case "ELSE" ~ block ⇒ Block(block._2) }
override def grammar = log(ifExpression)("ifexpression") ~ log(elseIFExpression)("elseifexpression") ~ log(elseExpression)("elseexpression") ^^ {
  case ifExpression ~ elseIFExpression ~ elseExpression ⇒
    ConditionalExpressions(List(ifExpression, elseIFExpression), elseExpression)
}

The above logic works after being evaluated like this -

object BasicSelectorExpressionEvaluator extends EvaluatorLike {

override def eval(parseable: Parseable) = parseable match {
  case ConditionalExpressions(ifElseIfs, otherwise) ⇒
    val mappedIfElseIfs: immutable.Seq[Block] = ifElseIfs.map { e ⇒
      println(s"e ==>$e")
      e.operator match {
        case "==" ⇒ if (e.lhs == e.rhs) {
          println("mached ==")
          Block(e.block)
        } else Block.Unit
        case "<" ⇒ if (e.lhs.value.toInt < e.rhs.value.toInt) {
          println("matched <")
          Block(e.block)
        } else Block.Unit
        case ">" ⇒ if (e.lhs.value.toInt > e.rhs.value.toInt) {
          println("matched >")
          Block(e.block)
        } else Block.Unit
        case "<=" ⇒ if (e.lhs.value.toInt <= e.rhs.value.toInt) {
          println("mached <=")
          Block(e.block)
        } else Block.Unit
        case ">=" ⇒ if (e.lhs.value.toInt >= e.rhs.value.toInt) {
          println("mached >=")
          Block(e.block)
        } else Block.Unit
      }
    }
    val filteredMappedIFElseIfs = mappedIfElseIfs.filterNot(e ⇒ e.equals(Block.Unit))
    println(s"filteredMappedIFElseIfs == $filteredMappedIFElseIfs")
    if (filteredMappedIFElseIfs.nonEmpty) PResult(filteredMappedIFElseIfs.head.block) else PResult(otherwise.block)
}

}

So the above can parse this grammar -

val s = s""" IF [Integer] $resultId == 100 GOTO NODE-1-->NODE-2 ELSE IF [Integer] $resultId > 100 GOTO NODE-1-->NODE-3 ELSE GOTO NODE-1-->NODE-4""".stripMargin

It could be done better, e.g. grammar seems to violate DRY by embedding data types on every If, but I suppose people can derive things out of it.

Edit - Also note - this toInt thing is a bit ugly, needs to be better designed, I will maybe post an update once I do so. I need to rework all grammar now that it all works - suggestions/improvements welcome, still learning.

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