为什么 Scala 没有 decltype? [英] Why Scala does not have a decltype?

问题描述

有时可能希望将 x 声明为与 y 相同的类型.使用 vals 类型推断可以很好地处理这个问题，但这在其他一些领域不起作用，比如 使用函数类型.

Sometimes one might want to declare x to be of the same type as y. With vals type inference handles this very well, but this does not work in some other areas, like with function types.

对于具有一些 C++ 经验的程序员来说似乎显而易见的解决方案是 decltype.当前的 Scala 中似乎没有这样的设施.

A solution which seems obvious to a programmer with some C++ experience would be a decltype. No such facility seems to be present in current Scala.

对链接问题的回答说明:

An answer to the linked questions tells:

因为类型不是一等公民

我不得不承认我不明白这一点.我不认为类型是 C++ 中的一等公民，但它仍然可以具有 decltype.对于泛型中的类型参数或类似的东西，我不会询问诸如 decltype 之类的东西(我知道泛型不是模板，类型在其中被删除).尽管如此，我认为一个允许我在需要类型的地方使用表达式类型的运算符 - 当然编译器必须能够评估表达式类型，否则 val 的类型推断定义是不可能的.

I have to admit I do not understand this. I do not think types are a first class citizens in C++, but still it can have the decltype. I am not asking about anything like decltype for type parameters in generics or anything like that (I understand generics are not templates and the types are erased in them). Still, I think an operator which would allow me to use a type of an expression in a place where a type is expected - certainly the compiler must be able to evaluate an expression type, otherwise type inference for val definition would not be possible.

decltype 可以像下面这样使用 - 代码没有尝试做任何有用的事情，只是为了说明语法和基本用法:

A decltype could be used like below - the code is not trying to do anything anything useful, just to illustrate the syntax and basic usage:

case class A(x:Int = 0)

val a = new A(10)
val b = new decltype(a)

def f(c:decltype(a)) : decltype(a.x+a.x)

缺少 decltype 是一个深思熟虑的决定，还是有一些特定的原因导致 Scala 不能拥有它?是否有一些使用编译时反射的解决方案可以实现这一点?

Is absence of decltype a deliberate decision, or are there some specific reasons why Scala cannot have it? Is there perhaps some solution using compile time reflection which would allow this?

推荐答案

我的第一次尝试:

class Decl[T] { type Type = T }
object Decl { def apply[T](x: T) = new Decl[T] }

例如，如果我们有一些变量 x 我们不想明确声明其类型:

For example, if we have some variable x whose type we don't want to state explicitly:

val d = Decl(x)
type TypeOfX = d.Type

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