unwrapping-optionals-the-quick-way-in-swift-expressive-flatmap.swift

let balanceToConvert: String? 
...
if let inputEntry = balance, inputEntry.sign == .negative {
    balanceToConvert = Int(inputEntry.input) // Can return nil
        .flatMap(Int.init)     // Convert to Int if not nil
        .flatMap { $0 * -1 }   // flip sign if not nil
        .flatMap(String.init)  // convert back to String if not nil
}
...