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#编程语言
#Rust

以前会争论类型标识符，应该放在变量前面还是后面好。

除了美感这种偏主观的判断以外，我在学了一点PLT皮毛之后，有一个新的视角：

类型标识符在变量前面的语言，大多是没有类型推断的，因为放在前面，所以无法省略，就是说：必须在声明这个变量的时候就（由程序员）确定变量的类型。这也有例外，比如C++后面引入的auto关键字，相当于给原先的语法打了一个“补丁”，通过这个关键字声明的变量就能支持类型推断。

反之，放在变量后面的类型标识符，因为是可以做到省略的，省略的时候就是类型推断发挥的空间了：推断出最适合的类型出来；同样也有例外，~~比如Go就不支持类型推断~~。

比如附图中的这段Rust代码：
同样的let a = 1，在两个上下文（context）里推断出来的是不同的类型：上面的代码由于没有别的信息，所以就选了i32类型；下面的代码由于要和已经声明为u64类型的b相加，所以推断出来a的类型为u64。

支持类型推断（type inference）的语言，（应该都）要做到类型安全（type safety）。比如：不能允许两个变量的数据，操作之后出现溢出等情况。

综上，我更喜欢支持类型推断、类型安全的语言。

Roman Elizarov（Project Lead for the Kotlin Programming Language）的一篇文章《Types are moving to the right》中也提到，21世纪之后才出现的编程语言，很多都是把类型标识符放在变量后面了，Go、Rust、Scala等。

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2.4K views老C, edited Mar 21, 2024 at 04:02

tg-me.com/codedump_notes/547

Create: 2024-03-21
Last Update: 2025-06-23 05:21:14

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