哈斯克爾誤差在約束非類型變量參數：民[C]

Question

當我嘗試使用ghci中載入我的代碼，它給了我這個錯誤：

Asig1.hs:4:1: 
    Non type-variable argument in the constraint: Num [c] 
    (Use FlexibleContexts to permit this) 
    When checking that ‘f’ has the inferred type 
     f :: forall c (t :: * -> *). 
      (Num c, Num [c], Foldable t) => 
      [c] -> [c] -> t [c] -> ([c], [c]) 

我不明白我做錯了什麼。 這是我的代碼：

module Asig1 where 

f as ys x = (s,z) 
     where 
     ws = zipWith (*) as ys 
     s = foldl (+) ws x 
     z = s 

Answer 1

因爲你使用foldl可能走錯了路。 foldl有簽名有：

Foldable t => (b -> a -> b) -> b -> t a -> b 

所以，你給它一個函數（這裏(+)）的初始值和值的序列。你或許可以解決你的代碼：

 
module Asig1 where 

f as ys x = (s,z) 
     where 
     ws = zipWith (*) as ys 
 s = foldl (+) x ws --instead of foldl (+) ws x 
     z = s 

可以進一步提高你的代碼，因爲z = s是沒有必要的：

 
module Asig1 where 

f as ys x = (s,s) 
     where 
     ws = zipWith (*) as ys 
     s = foldl (+) x ws --instead of foldl (+) ws x