[英]Using readMaybe to read list of natural numbers
I am using this function to read list of natural numbers from string in the following format: [1, 2, 3]
: 我正在使用此函数以以下格式从字符串中读取自然数列表:
[1, 2, 3]
:
readMaybeIntList :: String -> Maybe [Int]
readMaybeIntList line = case readMaybe line of
Just l -> return l
Nothing -> Nothing
Right now it only works for integers - what would be the correct way to check whether the numbers are natural? 现在,它仅适用于整数-检查数字是否自然的正确方法是什么? Should I modify the
Just l
clause to check whether all numbers are >=0
? 我是否应该修改
Just l
子句以检查所有数字是否均>=0
? Is it a good idea to return Nothing
from such nested Just
clause? 从嵌套的
Just
子句中返回Nothing
是个好主意吗?
You could use do
-notation and guard
from Control.Monad
to avoid the excessive pattern matching: 你可以用
do
-notation和guard
从Control.Monad
避免过度模式匹配:
import Text.Read
import Control.Monad
readMaybeNatural :: String -> Maybe Int
readMaybeNatural str = do
n <- readMaybe str
guard $ n >= 0
return n
readMaybeNaturals :: String -> Maybe [Int]
readMaybeNaturals =
sequence . map readMaybeNatural . words
Well, if you're going to use return
anyway to invoke the monad instance for Maybe
, then I think I'd probably write: 好吧,如果您仍然要使用
return
来为Maybe
调用monad实例,那么我想我可能会写:
import Text.Read
import Control.Monad
readMaybeNatList :: String -> Maybe [Int]
readMaybeNatList line = do
ns <- readMaybe line
guard $ all (>=0) ns
return ns
which is a more idiomatic application of the Maybe
monad. 这是
Maybe
monad的惯用用法。 Whether it's clearer than the explicit pattern-matching (and monad-free) alternative: 是否比显式模式匹配(和无monad)更清晰:
readMaybeNatList' :: String -> Maybe [Int]
readMaybeNatList' line =
case readMaybe line of
Just ns | all (>=0) ns -> Just ns
_ -> Nothing
is probably a matter of opinion and intended audience. 可能是意见和目标受众的问题。
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