一个validating credit card numbers 题目
要求输入是string，输出是Maybe【Int】因为如果是非数字的话就会反馈Nothing。
求问该怎么解决啊？

import Data.Char
text2digits :: [Char]-> Maybe [Int]
text2digits = mapM char2digit
char2digit :: Char -> Maybe Int
charToDigit ch = if isDigit ch then Just (digitToInt ch) else Nothing