Question

如何避免在IO中使用doesExist <- doesDirectoryExist path和case doesExist of ...等样板分配？

是否有比这更惯用的方式？

import System.Directory
import System.Environment

main = do
  path:_ <- getArgs
  doesDirectoryExist path >>= cond
    (putStrLn $ path ++ " Exists")
    (putStrLn $ path ++ " Does not exist")

cond b c a = if a then b else c

Answer 1

LambdaCase适用于此处：

{-# LANGUAGE LambdaCase #-}

import System.Directory
import System.Environment

main = do
  path:_ <- getArgs    

  doesDirectoryExist path >>= \case
    True -> putStrLn $ path ++ " Exists"
    _    -> putStrLn $ path ++ " Does not exist"

Answer 2

或者＆＃34;如果＆＃34;：

doesDirectoryExist path >>= \x ->
 if x then (putStrLn "Exists") else putStrLn ("Does not")

将匿名条件与IO混合的最简单方法

2 个答案: