用于理解/循环和类型化模式的 Scala [英] Scala for comprehensions/loops and typed patterns

问题描述

根据 Scala 语言规范的第 6.19 节，此 for 循环:

According to section 6.19 of the Scala Language Specification this for loop:

for (e <-p) e'

被翻译成:

p <- e.withFilter{case p => true; case _ => false}.foreach{case p => e′}

那么，为什么这个小程序:

So, why this small program:

object ForAndPatterns extends App {
  class A()
  class B() extends A

  val list: List[A] = List(new A(), new B(), new B())

  for {b: B <- list}
    println(b)
}

给出这个编译错误:

Error:(7, 13) type mismatch;
 found   : proves.ForAndPatterns.B => Unit
 required: proves.ForAndPatterns.A => ?
   for {b: B <- list}

当这个表达式:

list.withFilter{case a: B => true; case _ => false}.foreach{case b => println(b)}

不会出错.

推荐答案

你从规范中得到的翻译实际上是

The translation you get from specification is actually

list.withFilter{case b: B => true; case _ => false}.foreach{case b: B => println(b)}

但它仍然可以编译和工作.似乎 Scala 正在丢失 case 并转换为

but it still compiles and works. It seems like Scala is losing the case and translating to

list.withFilter{case b: B => true; case _ => false}.foreach{b: B => println(b)}

这会产生同样的错误.

事实证明这是一个已知的旧错误:https://github.com/scala/bug/issues/900.

This turns out to be a known and old bug: https://github.com/scala/bug/issues/900.

那里提供的解决方法:

object Typed { def unapply[A](a: A) = Some(a) }

for { Typed(b: B) <- list } println(b)

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