Haskell：显示和漂亮的实例 [英] Haskell: show and pretty-print instance

问题描述

我开始在大学开展智能系统研究，我们的第一语言是Haskell。我必须承认，到目前为止我还不是很熟悉它。本周部分任务是开发一个代数数据类型 Expr a ，它代表基本的算术运算（+， - ，*，/）
解决方案IMO应该是：

pre $ 模块表达式，其中

数据Expr a = Number a |
Var |
总和（Expr a）（Expr a）|
Prod（Expr a）（Expr a）|
Div（Expr a）（Expr a）|
Pot（Expr a）a
派生（显示）


到目前为止。目前的任务是为我们的函数实现一个漂亮的实例。即：

$ b

Plus（Pot（Var 2））（Num 3 ）

至

x ^ 2 + 3

所以，我不知道漂亮是什么意思。在搜索互联网后，我发现漂亮只意味着以人类可读形式重写输出。它是否正确？如果是这样，这对我的功能意味着什么？我是否必须用漂亮函数替换 show 函数？我不知道从哪里开始。

我在这里阅读了几个类似的问题，但没有在那里说明问题。我真的很高兴，如果有人能给我一些提示，建议，解决方案或其他任何东西！

解决方案

基本上，您只需要一个将 Expr a 转换为字符串：

  myPrettyPrint :: Expr a  - >字符串
  

无论你想要什么，都可以调用它，并且不要试图替换 show 。

为了实现此功能，您可能需要了解模式匹配。

I started Intelligent System studies at a university and our first language is Haskell. I must admit that I am not really familiar with it so far. Part of this week's task is to develop an algebraic datatype Expr a which represents the basic arithmetic operations (+,-,*,/).

The solution IMO should be:

module Expression where

    data Expr a = Number a |
    Var |
    Sum (Expr a) (Expr a) |
    Prod (Expr a) (Expr a) |
    Div (Expr a) (Expr a) |
    Pot (Expr a) a
            deriving (Show)     

Okay so far. The task is to implement a pretty-instance for our function now. i.e.:

from

Plus ( Pot ( Var 2)) ( Num 3)

to

x^2 + 3

So, I had no idea what "pretty" means. After searching the internet I found out that "pretty" only means to rewrite the output in a human readable form. Is this correct? If it is, what does it mean to my function? Do I have to replace the show function with a pretty function? I don't really know where to start.

I read several similar questions here but didn't get the point there. I would really be happy if someone could give me some hints, advice, solutions or whatever!

解决方案

Yes, that's what pretty print means.

Basically, you just need a function that converts an Expr a into a String:

myPrettyPrint :: Expr a -> String

Call it whatever you want, and don't try to replace show.

In order to implement this function, you'll probably want to learn about pattern matching.

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