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

问题描述

我试过寻找一个简单的解释，但我还没有找到一个。

编辑：
有人标记为家庭作业。我不再去上学了，我只是在学习Haskell，我正试图理解这个概念。纯粹是出于兴趣。

解决方案

简而言之，模式就像在数学中定义分段函数一样。您可以使用模式为不同的参数指定不同的函数体。当你调用一个函数时，通过比较实际参数和各种参数模式来选择合适的体。阅读 Haskell简介获取更多信息。

比较：

与等价的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成为Haskell版本中的一个守护程序，但其他两个条件仅仅是模式。模式是测试值和结构的条件，例如 x：xs ，（x，y，z）或只需x 。在分段定义中，基于 = 或∈关系的条件（基本上，说某事的条件是其他）变成模式。守卫允许更一般的条件。我们可以重写 fib 来使用警卫：

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

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

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

EDIT: 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.

解决方案

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.

Compare:

with the equivalent Haskell:

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

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)

这篇关于Haskell模式匹配 - 它是什么？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！