'Ambiguous reference to overloaded definition' when override method in derived class - scala

Question

I've got the following code:
import com.github.nscala_time.time.Imports._
class Account {
def balance(date: DateTime): Double = {
/* some logic that calculates balance on given date */
val calculatedBalance = 42
calculatedBalance
}
def balance: Double = balance(DateTime.now)
}
class Deposit(val interestRate: Double) extends Account {
override def balance(date: DateTime): Double = {
/* some logic that calculates balance for deposit account */
val calculatedBalance = 100 * interestRate;
calculatedBalance
}
}
I try to use these classes in the following way:
val simpleAccount = new Account
val depositAccount = new Deposit(0.1)
val simpleBalanceOnDate = simpleAccount.balance(DateTime.now + 1.month) // A
val depositBalanceOnDate = depositAccount.balance(DateTime.now + 1.month) // B
val simpleBalance = simpleAccount.balance // C
val depositBalance = depositAccount.balance // D
Cases A, B and C are compiled without any errors, but for the line D I see error message:
Error:(28, 38) ambiguous reference to overloaded definition,
both method balance in class Deposit of type (date: com.github.nscala_time.time.Imports.DateTime)Double
and method balance in class Account of type => Double
match expected type ?
val depositBalance = depositAccount.balance
^
Can you please explain why there is a compilation error in case D and why there are no in case C?
Thanks in advance!

Answer 1

I suppose the compiler is confused about parameterless method inheritance, although I can't explain why honestly, for a quick solution this should work:
class Account {
{ ... }
def balance(): Double = balance(DateTime.now)
}
val depositAccount = new Deposit(0.1)
val depositBalance = depositAccount.balance()
Why this happens is obscure to me, maybe somebody else knows how the scala compiler is seeing parameterless method inheritance.
Also reading around, specifically Programming in Scala:
Such parameterless methods are quite common in Scala. By contrast, methods defined with empty parentheses, such as def height(): Int, are called empty-paren methods. The recommended convention is to use a parameterless method whenever there are no parameters and the method accesses mutable state only by reading fields of the containing object (in particular, it does not change mutable state).
This convention supports the uniform access principle,1 which says that client code should not be affected by a decision to implement an attribute as a field or method. For instance, we could have chosen to implement width and height as fields instead of methods, simply by changing the def in each definition to a val:
abstract class Element {
def contents: Array[String]
val height = contents.length
val width =
if (height == 0) 0 else contents(0).length
}
The two pairs of definitions are completely equivalent from a client's point of view. The only difference is that field accesses might be slightly faster than method invocations, because the field values are pre-computed when the class is initialized, instead of being computed on each method call. On the other hand, the fields require extra memory space in each Element object. So it depends on the usage profile of a class whether an attribute is better represented as a field or method, and that usage profile might change over time. The point is that clients of the Element class should not be affected when its internal implementation changes.

Answer 2

Why don't you provide a default argument like this:
class Account {
def balance(date: DateTime = DateTime.now): Double = {
/* some logic that calculates balance on given date */
val calculatedBalance = 42
calculatedBalance
}
}
class Deposit(val interestRate: Double) extends Account {
override def balance(date: DateTime): Double = {
/* some logic that calculates balance for deposit account */
val calculatedBalance = 100 * interestRate;
calculatedBalance
}
}
val simpleBalanceOnDate = simpleAccount.balance(1) // A
val depositBalanceOnDate = depositAccount.balance(1) // B
val simpleBalance = simpleAccount.balance() // C
val depositBalance = depositAccount.balance() // D