iterate.swift

// Similar to `enumerate` but provides the collection's index type
// rather than an Int for the position
public func iterate<C: CollectionType>(collection: C) -> SequenceOf<(C.Index, C.Generator.Element)> {
    var index = collection.startIndex

    // type-inference doesn't want to work without this
    return SequenceOf { _ -> GeneratorOf<(C.Index, C.Generator.Element)> in
        return GeneratorOf {
            if index == collection.endIndex {
                return nil
            }

            let previous = index++
            return (previous, collection[previous])
        }
    }
}