如何將字符串轉換爲的也許詮釋

Question

我怎麼會去有關轉換的字符串這樣"13.2..2"到列表這樣

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

名單我已經在digitToInt一看，但它確實沒有照顧Maybe Int。有沒有辦法我可以修改digitToInt來處理Maybe Int？

Answer 1

如果希望所有非數字轉換爲Nothing，則CA氮利用警衛和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" 

使用衛士是，從我的非專業的認識，有點比使用if/else更地道。

Answer 2

您可以使用isDigit來測試digitToInt是否會成功。

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] 

我們可以引入一個新的函數清理它一點：

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

如果你是某個數據來絕對是一個數字或.（或者是高興異常被拋出，如果的數據是不同的），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"