Implementing an enum ForwardIndexType - swift

Question

I have been struggling to properly implement the ForwardIndexType protocol for an enum, in particular the handling of the end case (i.e for the last item without a successor). This protocol is not really covered in the Swift Language book.
Here is a simple example
enum ThreeWords : Int, ForwardIndexType {
case one=1, two, three
func successor() ->ThreeWords {
return ThreeWords(rawValue:self.rawValue + 1)!
}
}
The successor() function will return the next enumerator value, except for the last element, where it will fail with an exception, because there is no value after .three
The ForwardTypeProtocol does not allow successor() to return a conditional value, so there seems to be no way of signalling that there is no successor.
Now using this in a for loop to iterate over the closed range of all the possible values of an enum, one runs into a problem for the end case:
for word in ThreeWords.one...ThreeWords.three {
print(" \(word.rawValue)")
}
println()
//Crashes with the error:
fatal error: unexpectedly found nil while unwrapping an Optional value
Swift inexplicably calls the successor() function of the end value of the range, before executing the statements in the for loop. If the range is left half open ThreeWords.one..<ThreeWords.three then the code executes correctly, printing 1 2
If I modify the successor function so that it does not try to create a value larger than .three like this
func successor() ->ThreeWords {
if self == .three {
return .three
} else {
return ThreeWords(rawValue:self.rawValue + 1)!
}
}
Then the for loop does not crash, but it also misses the last iteration, printing the same as if the range was half open 1 2
My conclusion is that there is a bug in swift's for loop iteration; it should not call successor() on the end value of a closed range. Secondly, the ForwardIndexType ought to be able to return an optional, to be able to signal that there is no successor for a certain value.
Has anyone had more success with this protocol ?

Answer 1

Indeed, it seems that successor will be called on the last value.
You may wish to file a bug, but to work around this you could simply add a sentinel value to act as a successor.

Answer 2

It seems, ... operator
func ...<Pos : ForwardIndexType>(minimum: Pos, maximum: Pos) -> Range<Pos>
calls maximum.successor(). It constructs Range<T> like
Range(start: minimum, end: maximum.successor())
So, If you want to use enum as Range.Index, you have to define the next of the last value.
enum ThreeWords : Int, ForwardIndexType {
case one=1, two, three
case EXHAUST
func successor() ->ThreeWords {
return ThreeWords(rawValue:self.rawValue + 1) ?? ThreeWords.EXHAUST
}
}

Answer 3

This is an old Question but I would like to sum up some things and post another possible solution.
As #jtbandes and #rintaro already stated a closed range created with the start...end operator is internally created with start..<end.successor()
Afaik this is an intentional behavior in Swift.
In many cases you can also use an Interval where you thought about using a Range or where Swift declared a Range by default. The point here is that intervals aren't collections.
So this is not possible with Intervals
for word in ThreeWords.one...ThreeWords.three {...}
================
For the following I assume the snippet above was just a debug case to cross-check the values.
To declare an Interval you need to explicitly specify the type. Either a HalfOpenInterval (..<) or a ClosedInterval (...)
var interval:ClosedInterval = ThreeWords.one...ThreeWords.four
This requires you to make your enumeration Comparable. Although Int is Comparable already, you still need to add it to the inheritance list
enum ThreeWords : Int, ForwardIndexType, Comparable {
case one=1, two, three, four
func successor() ->ThreeWords {
return ThreeWords(rawValue:self.rawValue + 1)!
}
}
And finally the enumeration need to conform to Comparable. This is a generic approach since your enumeration also conforms to the protocol RawRepresentable
func <<T: RawRepresentable where T.RawValue: Comparable>(lhs: T, rhs: T) -> Bool {
return lhs.rawValue < rhs.rawValue
}
Like I wrote you can't iterate over it in a loop anymore, but you can have a quick cross-check using a switch:
var interval:ClosedInterval = ThreeWords.one...ThreeWords.four
switch(ThreeWords.four) {
case ThreeWords.one...ThreeWords.two:
print("contains one or two")
case let word where interval ~= word:
print("contains: \(word) with raw value: \(word.rawValue)")
default:
print("no case")
}
prints "contains: four with raw value: 4"