为什么收益不能与scala中的while循环一起使用 [英] why yield can not work with while loop in scala

问题描述

在Scala中，yield可以用于for循环；例如:

In Scala, yield can work with for-loops; for example:

val ints: IndexedSeq[Int] = for(i <- 1 to 10) yield i

但是我发现yield不能与while循环一起使用，例如像:

But I found that yield can not work with while-loops, e.g. like:

while (resultSet.next()) yield new Row(resultSet)

Scala为什么如此设计?

Why is Scala designed like this?

我已经在Google和stackoverflow上进行了搜索，但是找不到答案.

I have searched on Google and stackoverflow, but could not find an answer.

推荐答案

因为while循环是java的while循环，因此'for循环'转换为以下函数的调用:<IndexedSeq>.map(如果使用yield)或<IndexedSeq>.foreach(如果您不关心结果).

Because a while loop is a java equivalent while loop, and the 'for loop' is translated to function call of: <IndexedSeq>.map (if you use yield) or <IndexedSeq>.foreach (if you don't care the result).

Scala代码示例:

Example Scala Code:

class ForVsWhileLoop {
  val dummy = for(i <- 1 to 10) yield i

  var dummy2 = Seq.empty[Int]
  var i = 0
  while(i <= 10)
    dummy2 :+= i
}

编译为(scala -Xprint:parse ForVsWhileLoop.scala):

Compiles to (scala -Xprint:parse ForVsWhileLoop.scala):

[[syntax trees at end of                    parser]] // ForVsWhileLoop.scala
package <empty> {
  class ForVsWhileLoop extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };

    // ***********************************************
    // the 'for loop' is translated to a function call
    val dummy = 1.to(10).map(((i) => i));


    var dummy2 = Seq.empty[Int];
    var i = 0;

    // *******************
    // classic while loop
    while$1(){
      if (i.$less$eq(10))
        {
          dummy2.$colon$plus$eq(i);
          while$1()
        }
      else
        ()
    }
  }
}

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