创建一个可以同时包含int和字符串的类型 [英] Create a type that can contain an int and a string in either order

问题描述

我正在关注 Haskell简介，对于这个特殊的地方(用户定义的类型2.2)，我感到特别晦涩.到目前为止，我什至都不了解代码的哪一部分，以及作者的想法.(什么是 Pt -从来没有在任何地方定义它?).不用说，我无法执行/编译.

I'm following this introduction to Haskell, and this particular place (user defined types 2.2) I'm finding particularly obscure. To the point, I don't even understand what part of it is code, and what part is the thoughts of the author. (What is Pt - it is never defined anywhere?). Needless to say, I can't execute / compile it.

作为一个使我更容易理解的示例，我想定义一个类型，它是一对整数和一个字符串，或者一对字符串和一个整数，但没有别的.

As an example that would make it easier for me to understand, I wanted to define a type, which is a pair of an Integer and a String, or a String and an Integer, but nothing else.

将使用它的理论函数如下所示:

The theoretical function that would use it would look like so:

combine :: StringIntPair -> String
combine a b = (show a) ++ b
combine a b = a ++ (show b)

如果您需要有效的代码，也可以执行相同的操作，下面是执行此操作的CL代码:

If you need a working code, that does the same, here's CL code for doing it:

(defgeneric combine (a b)
  (:documentation "Combines strings and integers"))

(defmethod combine ((a string) (b integer))
  (concatenate 'string a (write-to-string b)))

(defmethod combine ((a integer) (b string))
  (concatenate 'string (write-to-string a) b))

(combine 100 "500")

推荐答案

这是定义数据类型的一种方法:

Here's one way to define the datatype:

data StringIntPair = StringInt String Int | 
                     IntString Int String 
    deriving (Show, Eq, Ord)

请注意，我已经为类型 StringIntPair 定义了两个构造函数，它们分别是 StringInt 和 IntString .

Note that I've defined two constructors for type StringIntPair, and they are StringInt and IntString.

现在定义 combine :

combine :: StringIntPair -> String
combine (StringInt s i) = s ++ (show i)
combine (IntString i s) = (show i) ++ s

我正在使用模式匹配来匹配构造函数并选择正确的行为.

I'm using pattern matching to match the constructors and select the correct behavior.

以下是一些用法示例:

*Main> let y = StringInt "abc" 123
*Main> let z = IntString 789 "a string"
*Main> combine y
"abc123"
*Main> combine z
"789a string"
*Main> :t y
y :: StringIntPair
*Main> :t z
z :: StringIntPair

有关示例的一些注意事项:

A few things to note about the examples:

• StringIntPair 是类型；在解释器中执行:t< expression> 会显示表达式的类型
• StringInt 和 IntString 是相同类型的构造函数
• 竖线( | )分隔构造函数
• 一个编写良好的函数应该匹配其参数类型的每个构造函数；这就是为什么我用两种模式编写 combine 的原因，每个构造函数一个模式

• StringIntPair is a type; doing :t <expression> in the interpreter shows the type of an expression
• StringInt and IntString are constructors of the same type
• the vertical bar (|) separates constructors
• a well-written function should match each constructor of its argument's types; that's why I've written combine with two patterns, one for each constructor

这篇关于创建一个可以同时包含int和字符串的类型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！