Using reduce() to build a dictionary in Swift - swift

Question

I'd like to build a dictionary using a functional programming style. My reduce() doesn't seem to work - I get a "fatal error: unexpectedly found nil while unwrapping an Optional value"
func loadMoveToCalendarsRules(calendarIndex: Int) -> [String]? {
// return something like ["phone call", "buzz", "ring"]
return NSUserDefaults.standardUserDefaults().objectForKey(generateMoveToCalendarsRules_NSUserDefaultsKey(calendarIndex)) as? [String]
}
// Add indeces to an array of any type
func addIndices<T>(toArray: [T]) -> [(index: Int, value: T)] {
return Array(zip(toArray.indices, toArray))
}
typealias CalendarRules = [EKCalendar : [String]]?
func buildCalendarRules(cals: [EKCalendar]) -> CalendarRules {
let sortedCals = cals.sort { $0.title.lowercaseString < $1.title.lowercaseString }
// build move to cal rules.
let indexedCalList = addIndices(sortedCals)
// go through the sorted calendars and build a dictionary that associates each calendar with a string array. (These are keywords that apply to the given calendar.)
let calendarRules = indexedCalList.reduce(nil as CalendarRules) {
accumulator, nextValue in
var retVal: [EKCalendar : [String]]? = accumulator
// if there are values found in NSUserDefaults for this calendar index then retrieve them.
if let rulesForCurrentCal = loadMoveToCalendarsRules(nextValue.index) {
retVal![nextValue.value] = rulesForCurrentCal // fatal error: unexpectedly found nil while unwrapping an Optional value
}
return retVal
}
print("------------ built calendar rules -------------")
print(Array(arrayLiteral: calendarRules?.keys))
print(Array(arrayLiteral: calendarRules?.values))
return calendarRules
}

Answer 1

Your retVal is optional, and starts as nil (the initial value you pass in), yet you are using retVal! to force-unwrap it. You could just use [:] (an empty dictionary) as the initial value, and then retVal wouldn't need to be optional at all.

Answer 2

You are starting with nil, and never instantiate a CalendarRules dictionary, so the attempt to performed a forced unwrapping of it with ! is going to fail. Instead, test to see if it's nil and if so, instantiate one.
Before I get to that, I'd first suggest defining calendar rules as a non-optional type. It makes things less confusing this way:
typealias CalendarRules = [EKCalendar : [String]]
Then, you could use nil-coalescing operator, ??, to instantiate the CalendarRules object when needed:
let calendarRules = indexedCalList.reduce(nil as CalendarRules?) { accumulator, nextValue in
if let rulesForCurrentCal = loadMoveToCalendarsRules(nextValue.index) {
var retVal = accumulator ?? CalendarRules()
retVal[nextValue.value] = rulesForCurrentCal
return retVal
}
return accumulator
}
It strikes me that there might be more efficient approaches, but this should address your "unexpectedly found nil" error.