Haskell中的类型比较 [英] Type comparison in Haskell

问题描述

我还在学习Haskell的基础知识，并且试图找到这个简单问题的答案，所以我提前道歉，因为我确信它很简单。

鉴于：

  data Fruit = Fruit | Apple | Orange 
 derived（Show，Eq）
 
a = Apple 
  

如何检查一个a是否为Fruit？

解决方案

假设您确实是指 type 比较，简单的答案是你不能。 Haskell是静态类型的，所以检查是在编译时完成的，而不是运行时完成的。所以，如果你有这样的功能：

  foo :: Fruit  - > Bool 
 foo Apple = True 
 foo x = False 
  

答案是否或者不是 x 是一个水果永远是是。

你可能试图做的是找到了解构建给定值的数据构造函数。要做到这一点，请使用模式匹配：

  fruitName :: Fruit  - > String 
 fruitName Fruit =Fruit
 fruitName Apple =Apple
 fruitName Orange =Orange
  

顺便说一下，如果您使用的是GHCi，并且您想知道某种类型的东西，请使用：t

 >让a = 123 
> ：t a 
 a :: Integer 
> 
  

I'm still just learning the basics of Haskell, and I've tried to find an answer to this simple question, so I apologize in advance, because I'm sure it's simple.

Given:

data Fruit = Fruit| Apple | Orange
    deriving (Show, Eq)

a = Apple

How do I check if some a is a Fruit?

解决方案

Assuming you really meant type comparison, the simple answer is "you can't". Haskell is statically typed, so the check is done at compile-time, not run-time. So, if you have a function like this:

foo :: Fruit -> Bool
foo Apple = True
foo x     = False

The answer of whether or not x is a Fruit will always be "yes".

What you might be trying to do is find out what data constructor a given value was constructed with. To do that, use pattern matching:

fruitName :: Fruit -> String
fruitName Fruit  = "Fruit"
fruitName Apple  = "Apple"
fruitName Orange = "Orange"

By the way, if you're using GHCi, and you want to know the type of something, use :t

> let a = 123
> :t a
a :: Integer
>

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