Object construction with validation in Scala, using that in an Anorm parser

Question

I have a simple case class Amount as below

case class Amount(value: Long, currency: Currency)

And an accompanying object to convert a string currency code into a Currency object

object Amount {
  private val log = Logger(getClass)
  def apply(value: Long, currencyCode: String) : Amount = {
    try {
      Amount(value, Currency.getInstance(currencyCode))
    } catch {
      case e: Exception =>
        log.error(s"Invalid currency code [$currencyCode]")
        throw new Exception(s"Invalid currency code [$currencyCode]")
    }
  }
}

Invocation :

val amount : Amount = Amount(1234, "USD")

When I read some data from the database, I have a custom parser such as

implicit val amountParser = Macro.parser[Amount]("value", "currencyCode")

However, the compiler complains

scala.ScalaReflectionException: value apply encapsulates multiple overloaded alternatives and cannot be treated as a method. Consider invoking `<offending symbol>.asTerm.alternatives` and manually picking the required method
[error]     at scala.reflect.api.Symbols$SymbolApi$class.asMethod(Symbols.scala:228)
[error]     at scala.reflect.internal.Symbols$SymbolContextApiImpl.asMethod(Symbols.scala:84)
[error]     at anorm.Macro$.parserImpl(Macro.scala:70)
[error]     at anorm.Macro$.namedParserImpl_(Macro.scala:25)
[error]     implicit val amountParser = Macro.parser[Amount]("value", "currencyCode")

How do I make this work ?

UPDATE

After understanding the response from @MikeAllen, I decided to leave the case class Amount and the object Amount as is, instead I wrote a custom parser for the Amount as below

    implicit private val amountParser = for {
        value <- long("value")
        currencyCode <- str("currency_code")
      } yield { 
           Amount(value, currencyCode) 
      }

Answer 1

The Scala compiler will automatically generate an Amount.apply factory method for creating case class instances, which is why you're getting this error - because you have multiple Amount.apply methods. One of these takes arguments of type ( Long , Currency ) and the other takes arguments of type ( Long , String ). The error message suggests that you need to select one of these from the overloaded alternatives reported through reflection .

Alternatively, your case class and companion might be reworked as follows:

final case class Amount(value: Long, currencyCode: String) {

  /** Currency. Will create an exception on instantiation if code is invalid. */
  val currency: Currency = {
    try {
      Currency.getInstance(currencyCode)
    }
    catch {
      case e: Exception =>
        Amount.log.error(s"Invalid currency code [$currencyCode]")
        throw new Exception(s"Invalid currency code [$currencyCode]")
    }
  }
}

object Amount {
  private val log = Logger(getClass)
}

This is not quite as elegant, admittedly, as you now have a field, currency , that isn't one of the case class's parameters and that isn't available for pattern matching, while also carrying around the string form.

A better solution would be to keep your original case class and convert the currency code field from a String into a Currency , before creating the Amount instance, as part of the parser:

val amountMapping = {
  get[Long]("value") ~ get[String]("currencyCode") map {
    case value ~ currencyCode => {
      val currency = {
        try {
          Currency.getInstance(currencyCode)
        }
        catch {
          case e: Exception =>
            Amount.log.error(s"Invalid currency code [$currencyCode]")
            throw new Exception(s"Invalid currency code [$currencyCode]")
        }
      }
      Amount(value, currency)
    }
  }
}

You can then use this to parse rows, for example with:

def amounts(): List[Amount] = SQL("select * from amounts").as(amountMapping *)