Question

I’m new to Scala but I was wondering it it is possible to implement a simple Equation parser in the language.

Say I have a few functions (much like Excel functions):

IF(Cond a=b, val_true, val_false)

MID(String, Start_pos, num_chars) – string extract

LEN(String) – length of a string

OR(cond1, cond2, ... condn)

AND(cond1, cond2, ... condn)

So the idea would be I could pass in a formula at runtime as a string from a user as a command line argument along with any other params say IF(LEN(param1)=4,MID(param1,2,1), MID(param1,0,LEN(param1)))

The idea is to evaluate the function, so if the user provides that above formula and the string “scat” then the output would be “a”. If the string “scala” was given then the output would be “scala”…

How easy would this be to implement in Scala? What is the best design approach? I know there are no function pointers (in C I would have parsed the formula string into a collection of func points and gone from there)…

Any advice on how to approach this in efficient Scala style would be appreciated.

Cheers!

Answer 1

This question has motivated to experiment with combinator parsers. Given the following algebraic data types representing a subset of your expressions:

import scala.util.parsing.combinator._
object Expr { type VARS = Map[String, Any] }
import Expr._
sealed trait Expr { def eval(v: VARS) : Any } 

case class If(cond: Cond, ifTrue: Expr, ifFalse: Expr) extends Expr {
  def eval(v: VARS) = 
    if (cond.eval(v)) ifTrue.eval(v) else ifFalse.eval(v)
}
case class Cond(left: Expr, right: Expr) extends Expr {
  def eval(v: VARS) = left.eval(v) == right.eval(v)
}
case class Len(ident: String) extends Expr { 
  def eval(v: VARS) = v(ident).toString.size
}
case class Mid(ident: String, start: Expr, count: Expr) extends Expr {
  def eval(v: VARS) = {
    val s = start.eval(v).asInstanceOf[Int]
    val e = s + count.eval(v).asInstanceOf[Int]
    v(ident).asInstanceOf[String].substring(s, e)
  }
}
case class Ident(ident: String) extends Expr   { def eval(v:VARS) = v(ident) }
case class StringLit(value: String) extends Expr { def eval(v:VARS) = value }
case class Number(value: String) extends Expr  { def eval(v:VARS) = value.toInt }

The following parser definition will parse your given expression and return a Expr object:

class Equation extends JavaTokenParsers {
  def IF: Parser[If] = "IF" ~ "(" ~ booleanExpr ~","~ expr ~","~ expr ~ ")" ^^ {
    case "IF" ~ "(" ~ booleanExpr ~ "," ~ ifTrue ~ "," ~ ifFalse ~ ")" =>
      If(booleanExpr, ifTrue, ifFalse)
  }
  def LEN: Parser[Len] = "LEN" ~> "(" ~> ident <~ ")" ^^ (Len(_))
  def MID: Parser[Mid] = "MID" ~ "(" ~ ident ~ "," ~ expr ~ "," ~ expr ~ ")" ^^ {
    case "MID" ~ "(" ~ ident ~ "," ~ expr1 ~ "," ~ expr2 ~ ")" =>
      Mid(ident, expr1, expr2) 
  }
  def expr: Parser[Expr] = (
    stringLiteral ^^ (StringLit(_))
    | wholeNumber  ^^ (Number(_))
    | LEN
    | MID 
    | IF 
    | ident ^^ (Ident(_))
  )
  def booleanExpr: Parser[Cond] = expr ~ "=" ~ expr ^^ {
    case expr1 ~ "=" ~ expr2 => Cond(expr1, expr2)
  }
}

Then parsing and evaluating the results can be done like this:

val equation = new Equation
val parsed = equation.parseAll(equation.expr,
   """IF(LEN(param1)=4,MID(param1,2,1), MID(param1,0,LEN(param1)))""")
parsed match {
  case equation.Success(expr, _) =>
    println(expr)
    // If(Cond(Len(param1),Number(4)),
    //   Mid(param1,Number(2),Number(1)),
    //   Mid(param1,Number(0),Len(param1)))
    println(expr.eval(Map("param1" -> "scala"))) // prints scala
    println(expr.eval(Map("param1" -> "scat")))  // prints a
  case _ =>
    println("cannot parse")
}

Note that the grammar I provided is just the minimum to make your example parse and there absolutely no error management or type checking. Process wise, I first came up with a grammar without the production ^^ ... that would parse your example, then added the Expr types but without the eval method, then the production ^^ ..., then I finally added the eval methods to the Expr trait and sub-classes.