scala Enumeration with constructor and lookup table

0
votes

I saw the following solution for an enum somewhere

object Day {
  val values = new ArrayBuffer[Day]
  case class Day(name:String) {
    values += this
  }
  val MONDAY = new Day("monday")
  val TUESDAY = new Day("tuesday")
}

This demonstrates what I am trying to go for EXCEPT there is a var hidden in the ArrayBuffer....that is kind of icky.

What I really want is a val lookupTable = Map() where when a request comes in, I can lookup "monday" and translate it to my enum MONDAY and use the enum throughout the software. How is this typically done. I saw sealed traits but didn't see a way to automatically make sure that when someone adds a class that extends it, that it would automatically be added to the lookup table. Is there a way to create a scala enum with lookup table?

A scala Enumeration seems close as it has a values() method but I don't see how to pass in the strings representing the days which is what we receive from our user and then translate that into an enum.

thanks, Dean

2
scalascala-2.10
Is not withName method what you want? scala-lang.org/api/current/index.html#scala.Enumeration - Krever
Will you be creating new versions of the Day type at runtime or is this going to be a kind of enum? If an enum, have you seen this question about macro-based sealed trait enums or this one? - Sean Vieira
withName scans the values of the enumeration looking for a match, so there's benefit to creating a lookup table. - cbmanica

2 Answers

3
votes

Updated to show you how to rename an enum val, add additional constructor parameters, and how to add additional methods to your enum.

object Currency extends Enumeration {

  sealed case class CurrencyVal(name: String, value: Int) extends Val(name) {
    def *(n: Int): Int = n * value
  }

  val PENNY   = CurrencyVal("penny", 1)
  val NICKLE  = CurrencyVal("nickle", 5)
  val DIME    = CurrencyVal("dime", 10)
  val QUARTER = CurrencyVal("quarter", 25)

  override def withName(name: String): CurrencyVal = {
    super.withName(name).asInstanceOf[CurrencyVal]
  }
}

If you don't override withName then you get back a Currency.Value and not a Currency.CurrencyVal.

0
votes

I saw sealed traits but didn't see a way to automatically make sure that when someone adds a class that extends it, that it would automatically be added to the lookup table. Is there a way to create a scala enum with lookup table

I made a small lib here that might help, called Enumeratum. Its usage is as follows:

import enumeratum._

// entryName is for serdes, customise it as you'd like
sealed abstract class Day(val entryName: String) extends EnumEntry

case object Day extends Enum[Day] {

   // This will add Day entries declared below into a map at compile time, automagically
  val values = findValues

  case object MONDAY extends Day("monday")
  case object TUESDAY extends Day("tuesday")
  // etc
}

Day.withName("monday") // => returns MONDAY