Question

imP:: Int -> IO String    
imP n = do
x <- getLine
if n >= 0 then return ( (concat (replicate n " ")) ++ fun1 x) else return ( fun2 n x)
where
    fun1 [] = ""
    fun1 (x:xs)
        | isAlpha x = [x] ++ fun1 xs
        | otherwise = "\n" ++ fun1 xs
    fun2 n [] = ""
    fun2 n (x:xs)
        | isAlpha x = [x] ++ fun2 n xs
        | otherwise = "\n" ++ (concat (replicate (abs n) " ")) ++ fun2 n xs

我有这个代码。给getLine输入“hello3mello”它返回：

"hello\nmello"

但我需要：

"hello
 mello"

编辑：

<interactive>:32:9:                                                                                                                                               
Couldn't match type `IO String' with `[Char]'                                                                                                                 
Expected type: String                                                                                                                                         
  Actual type: IO String                                                                                                                                      
In the first argument of `putStr', namely `(imP 3)'                                                                                                           
In the expression: putStr (imP 3)

Answer 1

putStr的类型为String -> IO ()，您无法将其应用于imP 3 :: IO String，因为putStr期望纯String而不是IO String }。这正是GHC的错误消息报告的内容。

我假设你不熟悉monad，所以我建议你阅读许多教程中的任何一个。在此期间，请使用\x -> imP x >>= putStr

Haskell“\ n”在IO字符串

1 个答案: