Adding map to Range in Swift

Question

I'm trying to extend Range with a map function similar to the one on Array:

extension Range {
    func map<T>(@noescape transform: (Element) -> T) -> [T] {
        var result = Array<T>()
        for i in self {
            result.append(transform(i))
        }
        return result
    }
}

Element is defined by Range:

struct Range<Element : ForwardIndexType> : …

---

I thought this looked fairly good, however when using it I get a compiler error:

let cellSubtitles: [String?] = {  // <-- Unable to infer closure type in the current context
    return 0...42.map {
        let week: Int = $0/7
        switch week {
        case 0:  return nil
        case 1:  return "Last Week"
        default: return "\(week) Weeks Ago"
        }
    }
}()

Note that the error is for the surrounding closure, not for the one passed to map.

Also, Changing the second line above to

return (0...42 as Range<Int>).map {

gets rid of the error.

I do not understand this as my map function should return [String?] in both cases. And anyways, I would have assumed that 0...42 is Range<Int> also without the cast.

I'm using Swift 2.0.

Answer 1

If you are using Swift 2.0 then map is already defined for Range since Range conforms to CollectionType, which implements map, so there is no need to define it yourself.