實例和類哈斯克爾

Question

我在Haskell下面的代碼：

module Shape where 
type Height = Float 
type Width = Float 
type Radius = Float 
data Rectangle = Rectangle Height Width 
data Circle = Circle Radius 

class (Eq a, Show a) => Shape a where 
    area :: a -> Float 
    perimeter :: a -> Float 

instance Shape Rectangle where 
    area (Rectangle h w) = h * w 
    perimeter (Rectangle h w) = h*2 + w*2 
    show (Rectangle h w) = "rectangle " ++ (show h) ++ " " ++ (show w) 
    (==) (Rectangle h w) (Rectangle c d) = (h == c) && (w == d) 


instance Shape Circle where 
    area (Circle r) = pi * r**2 
    perimeter (Circle r) = 2 * pi * r 
    show (Circle r) = "circle " ++ (show r) 
    (==) (Circle r) (Circle x) = r == x 

，我想補充展和公式的情況下，以確定新的情況下（如矩形* ==矩形* *）但我得到以下問題：

[1 of 1] Compiling Shape   (Shape.hs, interpreted) 
Shape.hs:24:17: `show' is not a (visible) method of class `Shape' 
Shape.hs:25:17: `==' is not a (visible) method of class `Shape' 
Shape.hs:31:12: `show' is not a (visible) method of class `Shape' 
Shape.hs:32:12: `==' is not a (visible) method of class `Shape' 
Failed, modules loaded: none. 

這是什麼意思？我怎樣才能使它工作？

---

編輯： 現在的代碼： （線13至31）

instance Eq Rectangle where 
    (Rectangle h w) == (Rectangle c d) = (h == c) && (w == d) 

instance Show Rectangle where 
    show (Rectangle h w) = "rectangle " ++ (show h) ++ " " ++ (show w) 

instance Shape Rectangle where 
    area (Rectangle h w) = h * w 
    perimeter (Rectangle h w) = h*2 + w*2 

instance Eq Circle where 
    (Circle r) == (Circle x) = r == x 

instance Show Circle where 
    show (Circle r) = "circle " ++ (show r) 

instance Shape Circle where 
    area (Circle r) = pi * r**2 
    perimeter (Circle r) = 2 * pi * r 

錯誤：

[1 of 1] Compiling Shape   (Shape.hs, interpreted) 
Shape.hs:19:5: parse error on input `instance' 
Failed, modules loaded: none. 

Answer 1

拆分出每種類型的實例，並且每個類型類它是一個實例。

instance Eq Rectangle where 
    (==) (Rectangle h w) (Rectangle c d) = (h == c) && (w == d) 
instance Show Rectangle where 
    show (Rectangle h w) = "rectangle " ++ (show h) ++ " " ++ (show w) 
instance Shape Rectangle where 
    area (Rectangle h w) = h * w 
    perimeter (Rectangle h w) = h*2 + w*2 

這只是每種類型的show和==定義不是Shape實例的一部分，他們爲Show和Eq這恰好是Shape依賴類型類實例的一部分。