類型錯誤

Question

我試圖創建自己的字符串隨機播放功能：

import System.Random 

-- usage case: my_shuffle "something" "" 

my_shuffle :: [Char] -> [Char] -> [Char] 
my_shuffle [] result = result 
my_shuffle s result = do 
    pos <- randomRIO (1, length s) 
    my_shuffle (remove_char pos) (result ++ (get_char pos)) 

get_char :: [Char] -> Int -> Char 
get_char s pos = s !! (pos - 1) 

remove_char :: [Char] -> Int -> [Char] 
remove_char s pos = take (pos - 1) s ++ drop pos s 

它返回的錯誤信息：

substitution_cipher.hs:8:16: 
    Couldn't match expected type `[t0]' with actual type `IO a0' 
    In the return type of a call of `randomRIO' 
    In a stmt of a 'do' expression: pos <- randomRIO (1, length s) 
    In the expression: 
     do { pos <- randomRIO (1, length s); 
      my_shuffle (remove_char pos) (result ++ (get_char pos)) } 

當我看到它關係到IO，但我不知道如何解決它。

Answer 1

首先，您沒有將字符串參數傳遞給remove_char和get_char。此外，您需要將get_char的結果轉換爲列表才能使用++。到my_shuffle遞歸調用應該像這樣：

my_shuffle (remove_char s pos) (result ++ [get_char s pos]) 

其次，您需要使用IO單子爲randomIO，所以my_shuffle簽名應該是：

my_shuffle :: [Char] -> [Char] -> IO [Char] 

然後最後你需要使用return在基本情況下（因爲你需要返回IO [Char]）：

my_shuffle [] result = return result 

隨着修復AP合股：

import System.Random 

my_shuffle :: [Char] -> [Char] -> IO [Char] 
my_shuffle [] result = return result 
my_shuffle s result = do 
    pos <- randomRIO (1, length s) 
    my_shuffle (remove_char s pos) (result ++ [get_char s pos]) 

get_char :: [Char] -> Int -> Char 
get_char s pos = s !! (pos - 1) 

remove_char :: [Char] -> Int -> [Char] 
remove_char s pos = take (pos - 1) s ++ drop pos s