无论是过滤器还是filterM -布尔值与IO布尔值的差异
提问于 2019-09-15 17:53:03
我正在尝试过滤掉那些包含斜杠的字符串：
import Data.List
import Control.Monad(filterM)
hasSlash :: [Char] -> Bool
hasSlash firline = do
isInfixOf "/" firline
main :: IO ()
main = do
let d = ["abcd","abc/d","a/bcd","abcd","ab/cd"]
-- filter out those that do not have '/'
e <- filterM hasSlash d
print e但是，我得到了以下错误：
soq_filter.hs:13:18: error:
• Couldn't match type ‘Bool’ with ‘IO Bool’
Expected type: [Char] -> IO Bool
Actual type: [Char] -> Bool
• In the first argument of ‘filterM’, namely ‘hasSlash’
In a stmt of a 'do' block: e <- filterM hasSlash d
In the expression:
do { let d = ...;
e <- filterM hasSlash d;
print e }问题在哪里?如何解决？谢谢。
回答 1
Stack Overflow用户
回答已采纳
发布于 2019-09-15 17:55:33
您不需要也不想要IO或任何其他monad来进行过滤。改为执行以下操作：
import Data.List
hasSlash :: [Char] -> Bool
hasSlash firline = isInfixOf "/" firline
main :: IO ()
main = do
let d = ["abcd","abc/d","a/bcd","abcd","ab/cd"]
-- filter out those that do not have '/'
let e = filter hasSlash d
print e请注意，为了避免需要一元上下文，使用了let e =而不是e <-。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/57946743
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