Class Haskell中的实例和类

我在Haskell中有以下代码：

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

我想添加Show和Eq的实例，以便定义新的案例（例如矩形**==矩形**），但我遇到了以下问题：

[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.

这意味着什么？我怎样才能让它工作呢

---

编辑： 现在的代码是： （第13至31行）

错误：

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

---

将每个类型及其实例所属的每个类型类的实例分开

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

---

只是每个类型的

show

和

=

定义不是

Shape

实例的一部分，它们是

show

和

Eq

的typeclass实例的一部分，恰好是

Shape

的依赖项，我完全按照你说的那样更改了它，我得到了这个错误：“[1/1]编译Shape（Shape.hs，已解释）Shape.hs:19:5:输入'instance'的解析错误失败，加载的模块：无。'哪一行是第19行？什么在它之前？这表明缺少括号或其他语法错误。@xBlackout我对该代码没有任何问题，我不确定发生了什么事？这个问题与制表有关，而不是与代码错误有关。谢谢大家!

Float

实际上不是

Eq

的合法实例。这与你目前的问题无关，但很可能会给你带来麻烦。如果您切换到行为更好的类型，如

Rational

（从

Data.Ratio

），通常会遇到更少的gotcha（但不能求平方根等）。如果您想定义“标准”实例，不要忘了尝试

派生（Show，Eq）

。对于定制的，您必须编写自己的代码。

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