doubleEveryOtherHelper :: [Integer] -> Int ->  [Integer]
doubleEveryOtherHelper ((x:x1:xs) len)
    | len > 2   = x : 2*x1 : doubleEveryOtherHelper(xs (len - 1))
    | len == 2 = x : 2*x1
    | len == 1 = [2*x]
    | otherwise = []

Parse error in pattern: (x : x1 : xs)