getter.scala

val m1: Money = ...
val m2: Money = ...
// 貧血症ドメインモデルはコードの意図が読み取りづらい
m1.checkHasSameCurrencyAs(m２)
val m3: Money = Money.adjustBy(m1.amount + m2.amount, m1.currency)
// 表現力の高いドメインモデルは設計の意図を明白にする
val m4: Money = m1.plus(m2) // もしくは m1 + m2 と記述

Money.scala

package domain

trait Money {
  
  def plus(other: Money): Money
  
  def minus(other: Money): Money
  
  // ...
}

MoneyImpl.scala

package domain

import java.util.Currency

final class MoneyImpl(amount: BigDecimal,
                      currency: Currency) extends Money {
  
  require(amount.scale == currency.getDefaultFractionDigits,
    "Scale of amount does not match currency")

  override def plus(other: Money): Money = {
    checkHasSameCurrencyAs(other)
    Money.adjustBy(amount + other.amount, currency)
  }

  override def minus(other: Money): Money = plus(other.negated)

}