Useful Lenses

traversal.scala

class BalanceSheet {
  val froms: List[Transaction]
  val toss: List[Transaction]
  /* ... */
}

class Transaction {
  val issue: DateTime
  val acknowledge: DateTime
  /* ... */
}

val sheetTransactionsLens: Traversal[BalanceSheet, Transaction] = 
  Traversal.applyN(
    GenLens[BalanceSheet](_.froms),
    GenLens[BalanceSheet](_.tos),
  )

val transactionTimesLens: Traversal[Transaction, DateTime] = 
  Traversal.applyN(
    GenLens[Transaction](_.isue),
    GenLens[Transaction](_.aknowledge),
  )

val sheetTimes: Traversal[BalanceSheet, DateTime] = 
  sheetTransactionsLens
  .composeTraversal(each)
  .composeTraversal(transactionTimesLens)

usage.scala

// We can re-use a simple function that goes from Time to a new Time
val adjustTime: DateTime => DateTime = ???
    
// And use the lense to build a function that goes from BalanceSheet to a new BalanceSheet.
val adjustSheetTimes: BalanceSheet => BalanceSheet = 
  sheetTimes.modify(adjustTime)

// This is a concrete alternative to adjustTime, we just increment it by duration:
def increaseTimeBy(duration: FiniteDuration): DateTime => DateTime = 
  _.add(duration)

// and re-use the same lense, to update the time on on transactions in a balance sheet adding to them a fixed amount.
def increaseSheetTimes(duration: FiniteDuration): BalanceSheet => BalanceSheet = 
  sheetTimes.modify(increaseTimeBy(duration))