在 for 循环和条件语句中添加到 Scala 映射 [英] Adding to scala map within for loop and conditional statement

问题描述

我收到一条错误消息错误:类型参数 [Any] 不符合 trait Cloneable 的类型参数边界 [+A <: AnyRef]"，我无法做出正面或反面.

I'm getting an error message of "error: type arguments [Any] do not conform to trait Cloneable's type parameter bounds [+A <: AnyRef]," which I can't make heads or tails of.

具体来说，

var M = mutable.Map[Int, mutable.Set[Int]]()
for(i <- 1 to 100; j <- 1 to 100) {
    if(!M.contains(i)) {M += i -> mutable.Set[Int](j)}
    else {M(i) += j} 
}

(我实际上正在尝试更复杂的东西，但这是经过调整并简化到最低限度的错误生成代码)

(I'm actually trying something more complicated, but this is the error generating code tweaked and simplified to a minimum)

上面代码的最后一行生成了错误信息.如果我进一步剥离它

And the last line of the above code generates the error message. If I strip it down further

for(i <- 1 to 100; j <- 1 to 100) {
    if(!M.contains(i)) {M += i -> mutable.Set[Int](j)}
}

它有效！

如何使上述代码工作?

推荐答案

Digal 诊断出问题(未能统一 if-else 分支的类型)，看起来像是编译器错误.这是一个进一步简化的情况，在经过漫长的编译时间后，它会在 REPL 中出现错误，

Digal diagnosed the problem (failure to unify the types of the if-else branches) and it looks like a compiler bug. Here's a further simplified case that will give an error in the REPL, after a lengthy compilation time,

if (true) {
  null: collection.mutable.Map[Int, Int]
} else {
  null: collection.mutable.Set[Int]
}

与此同时，您可以使用在 if-else 语句中的某处散布的显式类型来编译代码，

In the meantime, you can get your code to compile with an explicit type sprinkled somewhere in the if-else statement,

for(i <- 1 to 100; j <- 1 to 100) {
  if(!M.contains(i)) {M += i -> mutable.Set[Int](j)}
  else {M(i) += j}: Unit 
}

我在这里提交了一个问题:https://issues.scala-lang.org/浏览/SI-4938

I filed an issue here: https://issues.scala-lang.org/browse/SI-4938

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