如何将字符串转换为的也许诠释

Question

我怎么会去有关转换的字符串这样"13.2..2"到列表这样

[Just 1, Just 3, Nothing, Just 2, Nothing, Nothing, Just 2]

名单我已经在digitToInt一看，但它确实没有照顾Maybe Int。有没有办法我可以修改digitToInt来处理Maybe Int？

Answer 1

如果希望所有非数字转换为Nothing，则CA氮利用警卫和fmap

import Data.Char 

charToMaybeInt :: Char -> Maybe Int 
charToMaybeInt x 
| isDigit x = Just $ digitToInt x 
| otherwise = Nothing 

main = putStrLn $ show $ fmap charToMaybeInt "13.2..2" 

使用卫士是，从我的非专业的认识，有点比使用if/else更地道。

Answer 2

您可以使用isDigit来测试digitToInt是否会成功。

λ> fmap (\c -> if isDigit c then Just (digitToInt c) else Nothing) "13.2..2" :: [Maybe Int] 
[Just 1, Just 3, Nothing, Just 2, Nothing, Nothing, Just 2] 

我们可以引入一个新的函数清理它一点：

digitToIntMay :: Char -> Maybe Int 
digitToIntMay c = if isDigit c then Just (digitToInt c) else Nothing 

λ> fmap digitToIntMay "13.2..2" :: [Maybe Int] 
[Just 1, Just 3, Nothing, Just 2, Nothing, Nothing, Just 2] 

Answer 3

如果你是某个数据来绝对是一个数字或.（或者是高兴异常被抛出，如果的数据是不同的），可以使用模式匹配和fmap

import Data.Char 

chatToMaybeInt :: Char -> Maybe Int 
chatToMaybeInt '.' = Nothing 
chatToMaybeInt x = Just $ digitToInt x 

main = putStrLn $ show $ fmap chatToMaybeInt "13.2..2"