function - 为什么在函数定义中首选模式匹配？

Question

我正在阅读来自 learnyouahaskell 的“learnyouahaskell”教程.那里写着:

Pattern matching can also be used on tuples. What if we wanted to make a function that takes two vectors in a 2D space (that are in the form of pairs) and adds them together? To add together two vectors, we add their x components separately and then their y components separately. Here's how we would have done it if we didn't know about pattern matching:

addVectors :: (Num a) => (a, a) -> (a, a) -> (a, a)  
addVectors a b = (fst a + fst b, snd a + snd b)  

Well, that works, but there's a better way to do it. Let's modify the function so that it uses pattern matching.

addVectors :: (Num a) => (a, a) -> (a, a) -> (a, a)  
addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2)  

There we go! Much better. Note that this is already a catch-all pattern. The type of addVectors (in both cases) is addVectors :: (Num a) => (a, a) -> (a, a) - > (a, a), so we are guaranteed to get two pairs as parameters.

我的问题是:如果两个定义产生相同的签名，为什么模式匹配是首选方式？

Answer 1

我认为在这种情况下，模式匹配更直接地表达了你的意思。

在功能应用案例中，需要知道什么fst和 snd做，并从中推断出a和 b是添加元素的元组。

addVectors a b = (fst a + fst b, snd a + snd b)

我们有 snd 的事实和 fst分解元组的函数在这里分散注意力。

在模式匹配的情况下，输入是什么(一个元组，我们称之为 x1 和 y1 和一个元组......等)以及它是如何被解构的。并且它也立即清楚发生了什么，它们的元素是如何添加的。

addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2)

这几乎就像数学定义:

(x₁, y₁) + (x₂, y₂) := (x₁ + x₂, y₁ + y₂)

直截了当，没有分心:-)

你可以从字面上用 Haskell 写这个:

(x₁, y₁) `addVector` (x₂, y₂) = (x₁ + x₂, y₁ + y₂)