在Haskell

Question

使用括號的

我收到此錯誤：

<interactive>:145:29: 
    Could not deduce (Integral ([a0] -> Int)) 
     arising from a use of ‘fromIntegral’ 
    from the context (Num ([a] -> a), Fractional a) 
     bound by the inferred type of 
       meanList :: (Num ([a] -> a), Fractional a) => [a] -> a 
     at <interactive>:145:5-50 
    The type variable ‘a0’ is ambiguous 
    In the second argument of ‘(/)’, namely ‘(fromIntegral length x)’ 
    In the expression: (sum x)/(fromIntegral length x) 
    In an equation for ‘meanList’: 
     meanList x = (sum x)/(fromIntegral length x) 

由產生上述錯誤：

然而，當更新這：

let meanList x = sum x/fromIntegral (length x) 

然後，所有很好。

括號在Haskell中如何工作？

Answer 1

函數應用程序是關聯的。換句話說，

fromIntegral length x = (fromIntegral length) x 

因此，錯誤Could not deduce (Integral ([a0] -> Int))因爲長度的類型確實沒有的Integral一個實例。