xcode 为什么在 Swift 中用关键字“let”声明了一个常量？
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Why is a constant declared with the keyword "let" in Swift?
提问by theremin
the title says it all...Why is a constant declared with the keyword "let" in Swift?
标题说明了一切......为什么在 Swift 中用关键字“let”声明一个常量？
Probably there's a simple answer to this noob question, but I couldn't find it on SO.
这个菜鸟问题可能有一个简单的答案，但我在 SO 上找不到。
EDIT: OK, just to make the question clearer. I know that it needs to be initialized with SOME name, but I thought that there maybe is a deeper meaning to let, a source where it originates? Other stuff like "func" seems very logical to me, so I wonder what the deeper meaning of "let" is.
编辑：好的，只是为了让问题更清楚。我知道它需要使用 SOME 名称进行初始化，但我认为可能有更深层次的含义，它起源于何处？像“func”这样的其他东西对我来说似乎很合乎逻辑，所以我想知道“let”的深层含义是什么。
采纳答案by Antonio
Actually in swift there is no concept of constant variable.
实际上在 swift 中没有常量变量的概念。
A constant is an expression that is resolved at compilation time. For example, in objective C this code:
常量是在编译时解析的表达式。例如，在目标 C 中，此代码：
const NSString *string = [[NSString alloc] init];
results in a compilation error, stating that Initializer element is not a compile-time constant. The reason is that NSStringis instantiated at runtime, so it's not a compile time constant.
导致编译错误，说明Initializer element is not a compile-time constant. 原因是它NSString是在运行时实例化的，所以它不是编译时常量。
In swift the closest thing is the immutable variable. The difference may not be evident, but an immutable is not a constant, it's a variable that can be dynamicallyinitialized once and cannot be modified thereafter. So, the compile time evaluation is not needed nor required - although it will frequently happen we use immutables as constants:
在 swift 中最接近的是不可变变量。区别可能不明显，但不可变不是常量，它是一个变量，可以动态初始化一次，之后不能修改。因此，不需要也不需要编译时评估——尽管我们经常使用不可变变量作为常量：
let url = "http://www.myurl.com"
let maxValue = 500
let maxIntervalInSeconds = 5 * 60 *60
But immutables can also be initialized with expressions evaluated at runtime:
但是不可变也可以使用在运行时计算的表达式进行初始化：
let url = isDebug ? "http://localhost" : "http://www.myservice.com"
let returnCode: Int = {
switch(errorCode) {
case 0: return 0
default: return 1
}
}()
The latter example is interesting: using a closure, immediately executed, to initialize an immutable variable (differently from var, immutables don't support deferred initialization, so that's the only way to initialize using a multi line expression)
后一个例子很有趣：使用一个立即执行的闭包来初始化一个不可变变量（与 不同var，不可变不支持延迟初始化，所以这是使用多行表达式初始化的唯一方法）
