Reduce, without the initial value

gistfile1.txt

enum ListError: ErrorType {
    case Empty
    case Unknown
}

extension CollectionType {
    
    typealias Elem = Self.Generator.Element
    
    func list(combine:(Elem, Elem) -> Elem) throws -> Elem {
        var gen = self.generate()
        var cur:Elem? = gen.next()
        guard let first = cur else{
            throw ListError.Empty
        }
        var x:Elem = first
        cur = gen.next()
        
        while(cur != nil){
            x = combine(x,cur!)
            cur = gen.next()
        }
        
        return x
    }
    
    func list<T>(transform:(Elem -> T), @noescape combine:(T, T) -> T) throws -> T{
        var gen = self.generate()
        var cur = gen.next()
        guard let first = cur else{
            throw ListError.Empty
        }
        var x:T = transform(first)
        cur = gen.next()
        
        while(cur != nil){
            x = combine(x,transform(cur!))
            cur = gen.next()
        }
        return x
    }
}

let nums = [1,2,3,4,5,6,7,8]
let numb = try? nums.list(+)
print(numb)
//Optional(36)

let strNums = ["1","2","3","4","5","6","7","8"]
let strNumb = try? strNums.list{$0+","+$1}

print(strNumb)
//Optional("1,2,3,4,5,6,7,8")