Array of NSManagedObjects and reduce function compile but fail at runtime - core-data

Question

Using playground to simulate a problem with a core data based app turned up an issue I can't seem to understand.
class Pole {
var length: NSNumber!
}
var poles: [Pole] = []
let pole1 = Pole()
pole1.length = 1
poles.append(pole1)
let pole2 = Pole()
pole2.length = 2
poles.append(pole2)
var sum = poles.reduce(0) { $0 + $1.length } // error Could not find member 'length'
The property (attribute) named length is NSNumber as it is in a NSManagedObject class (entity).
Changing the Type from NSNumber! to Int! allows the line to compile and run correctly in playground.
Leaving the the Type as NSNumber! and changing the the offending line as follows:
var sum = poles.reduce(0) { $0 + Int($1.length) }
also compiles and run correctly in playground. The next step, taking this to the app, using actual NSManagedObject entity and attribute compiles but fails at runtime. The failure is 'unwrapping a nil'.
So bottom line I can't figure out how to use the reduce function when the attribute is a NSNumber and casting it to an Int doesn't seem to be acceptable.

Answer 1

NSNumber is a Foundation class that is used to wrap numbers that would otherwise be represented in a value type, so they can be used wherever a reference type is expected. Swift bridges integer literals to NSNumber, but not the other way around, so the original reduce closure was trying to add an integer literal to an NSNumber, getting confused, and giving you a weird error message.
Creating a new Int is one way to handle the problem:
var sum = poles.reduce(0) { $0 + Int($1.length) }
Or you can use NSNumber's integerValue property, which returns the instance's value as an Int:
var sum = poles.reduce(0) { $0 + $1.length.integerValue }