Question

I have a domain model that looks like this:

case class Account(id: Int, name: String)

trait BalanceTotal {
  def balance: BigDecimal
}

My goal is to have a simple, lightweight Account case class and then an enhanced class Account with BalanceTotal that will only be created inside a method that does the balance calculation (which is expensive).

With this structure, I can statically ensure that I’m never counting on a balance being present when I just have a lightweight object. I know about Option, of course, but I want the type-checker to prevent code from using the lightweight object where I need the balance-enriched object, and vice-versa, and I don’t think Option gives me that.

Anyway, what I’d like to do is:

for{ Account(id,name) <- accounts } {
  yield Account(id, name) with BalanceTotal {
    override val balance = BigDecimal(44)
  }
}

but that fails with:

';' expected but 'with' found

If I try to use new Account instead of Account, I get:

super constructor arguments cannot reference unconstructed `this`

which I assume has something to do with the voodoo that makes case classes work.

Is it possible to do what I’m trying to do?

Update: On further consideration, I guess this has something to do with the prohibition against case class inheritance. But I really do need case class magic here: I’m relying on the generated apply and unapply methods for my ScalaQuery mappings.

Answer 1

It works OK for me:

>scala
Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.
7.0).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

case class Account(id: Int, name: String)

trait BalanceTotal {
  def balance: BigDecimal
}
val enrichedAccount = new Account(1, "foo") with BalanceTotal {
  override val balance = BigDecimal(44)
}

// Exiting paste mode, now interpreting.

defined class Account
defined trait BalanceTotal
enrichedAccount: Account with BalanceTotal = Account(1,foo)