我想将 Maybe [int] 合并到其中。
代码应该采用一个字符串并过滤掉空格,将其转换为整数列表,如果它们是字母,则返回 Nothing。
text2digits :: String -> [Int]
text2digits s = case s of
[] -> []
x:xs
|isDigit x -> digitToInt x :text2digits (filter (/= ' ') xs)
|otherwise -> undefined
input "1233 5687" output: [1,2,3,3,5,6,8,7]
input "a89" required output : Nothing
current output: undefined
我已经尝试过,但它显示了错误列表
text2digits :: String -> Maybe [Int]
text2digits s = case s of
[] -> Just []
x:xs
|isDigit x -> Just digitToInt x :text2digits (filter (/= ' ') xs)
|otherwise -> Nothing
最佳答案
您为 text2digits :: String -> Maybe [Int]
指定的代码有什么问题? ?
问题出在这一行:
digitToInt x :text2digits (filter (/= ' ') xs)
text2digits
返回值 Maybe [Int]
类型,但是 (:)
预计为[Int]
.
为了修复它,您可以使用 fmap
或<$>
将函数应用于仿函数内的结构 Maybe
:
import Data.Char
text2digits :: String -> Maybe [Int]
text2digits s = case s of
[] -> Just []
x:xs
|isDigit x -> ((digitToInt x) :) <$> text2digits (filter (/= ' ') xs)
|otherwise -> Nothing
main = print $ text2digits "1233 5687"
或者您可以使用traverse稍微重构一下函数:
import Data.Char
text2digits :: String -> Maybe [Int]
text2digits s =
traverse digitToMaybeInt $ filter (/= ' ') s
where
digitToMaybeInt x
| isDigit x = Just $ digitToInt x
| otherwise = Nothing
main = print $ text2digits "89"
关于字符串到 Int 列表,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/46357690/