Scala 中的表达式和语句有什么区别 [英] What is the difference between Expressions and Statements in Scala

问题描述

我是 Scala 世界的新手，想知道表达式和语句之间有什么区别，以及为什么将 if-else 用于表达式而不是语句.如果有办法在 if-else 中使用语句?

I'm new in Scala world and want to know what is the difference between Expressions and Statements and Why if-else is used for expression, not statements. And if there is a way to use statement in if-else?

推荐答案

EDIT:正如@Jörg W Mittag 在评论中指出的那样，这个答案实际上是错误的:声明确实存在于 Scala 规范中:

EDIT: As pointed out by @Jörg W Mittag in the comments, this answer is in fact wrong: statements do exist in the Scala spec:

语句作为块和模板的一部分出现.语句可以是导入、定义或表达式，也可以为空.

Statements occur as parts of blocks and templates. A statement can be an import, a definition or an expression, or it can be empty.

我保留原始答案，因为我认为它作为对 OP 问题的实际答案仍然很有价值.

I leave the original answer, as I think it is still valuable as a practical answer to the OP's question.

原始答案

迂腐地说，Scala 中没有声明这样的东西.有定义(如def 和class)和表达式.在其他语言中被称为语句的一切都只是一个表达式.

Pedantically speaking, there is no such thing as a statement in Scala. There are definitions (such as def and class) and expressions. Everything that would be called a statement in other languages is just an expression.

例如，许多人会调用以下打印到控制台的行"，一个语句:

For example, many people would call the following "line", which prints to the console, a statement:

println("hello")

在 Scala 中，这是一个表达式.比如可以赋值给一个值:

In Scala, this is an expression. For example, it can be assigned to a value:

val x = println("hello")

在这种情况下，x 被分配了类型 Unit 的值 ()(读作 unit).

In this case, x is assigned the value () (pronounced unit) of type Unit.

除了理论"之外，通常的做法是调用始终返回 () 语句的表达式.这意味着上面的 println("hello") 也可以看作是一个语句.

"Theory" aside, it is common practice to call an expression that always returns () a statement. Which means that the above println("hello") can also be considered as a statement.

现在让我们回到if/else.由于一切都是表达式，所以这个 if/else 是一个表达式:

Let us now come back to if/else. Since everything is an expression, this if/else is an expression:

if (cond) 3 else 5

不过如此

if (cond)
  println("hello")
else
  println("bye")

在后一个例子中，if/else 总是返回 ()(因为 println 都返回 ())，因此您可以将此 if/else 称为语句.

In the latter example, though, the if/else always returns () (because both printlns return ()), and you can therefore call this if/else a statement.

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