显示多态类型的函数 [英] Show function for polymorphic type

问题描述

我试图为多态树类型定义Show函数。任何人都可以帮助我吗？

  import char 
 
数据树t = 
 NilT | 
 Node t（Tree t）（Tree t）
 
 class Mar t其中
 maior :: t  - >字符串
 
实例Mar Tree其中
 maior（NilT）=a
 maior（节点标签）=b
 
实例Show Tree where 
 show = maior 
  

非常感谢！

---

解决方案（由ivanm提供）：

  import Char 
 
数据树t = 
 NilT | 
 Node t（Tree t）（Tree t）
 
 class Mar t其中
 maior :: t  - >字符串
 
实例Mar（Tree t）其中
 maior（NilT）=a
 maior（节点标签）=b
 
实例Show（Tree t）where 
 show = maior 
  

解决方案

是否有任何特殊原因导致您没有使用派生Show ？ Show 和 Read 类旨在提供真正基本的序列化/反序列化，它通常会生成有效的Haskell代码。 / p>

但是对于你想要的，我认为这个错误会回到你的 Mar 类中。按照定义，该实例是用于 * - > * （例如 Maybe 而不是 Maybe Int ）。你可能意思是有实例Mar（Tree t）其中... 。

I'm trying to define the Show function for the polymorphic Tree type. Could anyone help me?

import Char

data Tree t =
    NilT |
    Node t (Tree t) (Tree t)

class Mar t where
    maior :: t -> String

instance Mar Tree where
    maior (NilT) = "a" 
    maior (Node t a b) = "b"

instance Show Tree where
    show = maior

Thanks a Lot!

---

Solution (given by ivanm):

import Char

data Tree t =
    NilT |
    Node t (Tree t) (Tree t)

class Mar t where
    maior :: t -> String

instance Mar (Tree t) where
    maior (NilT) = "a" 
    maior (Node t a b) = "b"

instance Show (Tree t) where
    show = maior

解决方案

Is there any particular reason that you aren't using deriving Show? The Show and Read classes are meant to provide really basic serialisation/deserialisation which (usually) produce valid Haskell code.

But for what you want, I think the error is going back to your Mar class. As defined, the instance is for kind * -> * (e.g. Maybe as opposed to Maybe Int). What you probably meant is to have instance Mar (Tree t) where ....

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