類型錯誤與分數相關

Question

我試圖找出最新的錯誤。似乎類型有問題，但單獨使用相同的表達方式可以正常工作。

下面是代碼：

a = [9, 4, 12, 0, -6, 16] :: [Int] 

qsort:: [Int] -> [Int] 
qsort [] = [] 
qsort [x] = [x] 
qsort xs = (qsort l)++(qsort r) 
    where m = (realToFrac(sum xs))/(realToFrac(length xs)) 
      l = filter (<=m) xs 
      r = filter (>m) xs 

main::IO() 
main = do 
     print (show (qsort a)) 

它拋出：

main.hs:7:36: 
    No instance for (Fractional Int) 
     arising from a use of `/' 
    Possible fix: add an instance declaration for (Fractional Int) 
    In the expression: (realToFrac (sum xs))/(realToFrac (length xs)) 
    In an equation for `m': 
     m = realToFrac (sum xs)/(realToFrac (length xs)) 
    In an equation for `qsort': 
     qsort xs 
      = (qsort l) ++ (qsort r) 
      where 
       m = realToFrac (sum xs)/(realToFrac (length xs)) 
       l = filter (<= m) xs 
       r = filter (> m) xs 

但是效果很好：

a = [9, 4, 12, 0, -6, 16] :: [Int] 
main::IO() 
main = do 
    print (show ( (realToFrac(sum a))/(realToFrac(length a)) )) 

Answer 1

你m定義給出了一些Fractional類型。 但是，您在l和r的定義中比較m到Int，這使混淆編譯器。 下面將做的伎倆：

a = [9, 4, 12, 0, -6, 16] :: [Int] 

qsort:: [Int] -> [Int] 
qsort [] = [] 
qsort [x] = [x] 
qsort xs = (qsort l)++(qsort r) 
    where m = realToFrac(sum xs)/(realToFrac(length xs)) 
      l = filter ((<=m) . fromIntegral) xs 
      r = filter ((>m) . fromIntegral) xs 

main::IO() 
main = do 
     print (show (qsort a)) 

Answer 2

realToFrac(sum xs)/(realToFrac(length xs))的工作，但它的結果是一個小數。所以在你的代碼m是一個分數。但是，您將m與您列表中的項目進行比較，這些項目是Ints。由於只能比較相同類型的值，並且Ints不是小數，因此會導致類型錯誤。

要解決您的問題，您可以使用整數除法（div）或將列表中的整數轉換爲小數，然後再將它們與m進行比較。

Answer 3

您可以使用x作爲您的支點。列表模式匹配「qsort（x：xs）」在這裏幫助很大。

a = [9, 4, 12, 0, -6, 16] :: [Int]                      

qsort:: [Int] -> [Int]                         
qsort [] = []                           
qsort (x:xs) = (qsort l) ++ [x] ++ (qsort r)                   
    where l = filter (<=x) xs                      
      r = filter (>x) xs                       

main::IO()                            
main = putStrLn $ show $ qsort a 

-- *Main> main                           
-- [-6,0,4,9,12,16] 

替代語法，以及：http://hpaste.org/81793