Haskell 模式匹配 - 它是什么? [英] Haskell pattern matching - what is it?

问题描述

Haskell 中的模式匹配是什么?它与保护方程有什么关系?

What is pattern matching in Haskell and how is it related to guarded equations?

我试图寻找一个简单的解释，但我没有找到.

I've tried looking for a simple explanation, but I haven't found one.

有人标记为家庭作业.我不再上学了，我只是在学习 Haskell，我正在努力理解这个概念.纯粹出于兴趣.

Someone tagged as homework. I don't go to school anymore, I'm just learning Haskell and I'm trying to understand this concept. Pure out of interest.

推荐答案

简而言之，模式就像定义数学中的分段函数.您可以使用模式为不同的参数指定不同的函数体.当您调用函数时，通过将实际参数与各种参数模式进行比较来选择适当的主体.阅读Haskell 简介了解更多信息.

In a nutshell, patterns are like defining piecewise functions in math. You can specify different function bodies for different arguments using patterns. When you call a function, the appropriate body is chosen by comparing the actual arguments with the various argument patterns. Read A Gentle Introduction to Haskell for more information.

比较:

使用等效的 Haskell:

with the equivalent Haskell:

fib 0 = 1
fib 1 = 1
fib n | n >= 2 
      = fib (n-1) + fib (n-2)

注意分段函数中的n ≥ 2"在Haskell版本中变成了守卫，但其他两个条件只是模式.模式是测试值和结构的条件，例如 x:xs、(x, y, z) 或 Just x.在分段定义中，基于 = 或 ∈ 关系的条件(基本上，说某物是"某物的条件)成为模式.警卫允许更一般的条件.我们可以重写 fib 来使用守卫:

Note the "n ≥ 2" in the piecewise function becomes a guard in the Haskell version, but the other two conditions are simply patterns. Patterns are conditions that test values and structure, such as x:xs, (x, y, z), or Just x. In a piecewise definition, conditions based on = or ∈ relations (basically, the conditions that say something "is" something else) become patterns. Guards allow for more general conditions. We could rewrite fib to use guards:

fib n | n == 0 = 1
      | n == 1 = 1
      | n >= 2 = fib (n-1) + fib (n-2)

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