If 语句作用域变量 [英] If-statement scoped variables

问题描述

我经常希望创建作用域为 if 语句的变量.一些计算只与特定的if"语句相关 - 用临时变量来污染外部作用域.

Often times I have a desire to create variables scoped to an if statement. Some computations only relate to a particular 'if' statement - to pollute the outer scope with temporary variables smells bad.

我想做什么:

val data = (whatever)

if (val x = data*2+5.4345/2.45; val y = data/128.4; x*y < 10) 
  x * y
else
  x * 2

println(x) //ERROR!

另一种选择相当混乱:

val data = (whatever)

if (data*2+5.4345/2.45*data/128.4 < 10) 
  data*2+5.4345/2.45*data/128.4
else
  data*2+5.4345/2.45 * 2

我试图避免的明显替代方案:

The obvious alternative I'm trying to avoid:

val data = (whatever)
val x = data*2+5.4345/2.45
val y = data/128.4

if (x*y < 10) 
  x*y
else
  x * 2

println(x) //OK

在 Scala 中可以实现这样的功能吗?有体面的解决方法吗?如果没有，还有哪些其他语言支持这样的想法?

Is something like this possible in Scala? Is there a decent workaround? If not, what other languages support an idea like this?

推荐答案

因为 if 在 Scala 中是一个表达式，即它返回一个值，通常你会为结果设置一些值你的 if 表达式.所以你的第三个选择很好:把它放在一个代码块中，即

Since if in Scala is an expression, i.e. it returns a value, normally you'd be setting some value to the result of your if expression. So your third alternative is just fine: put it in a code block, i.e.

val data = (whatever)
val myValue = {
  val x = data*2+5.4345/2.45
  val y = data/128.4

  if (x*y < 10) 
    x*y
  else
    x * 2
}

在块内声明的 val 在块外都不可用.

None of the vals declared within the block are available outside it.

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