我看了《Programming in Scala》一书，仍然对 Scala yield 关键字的理解不甚清楚。起初我以为 Scala yield 的与 Ruby 的 yield 是一样，Ruby 中 yield 是被传入代码块的占位符。Scala 中的 yield 关键字好像总是在 for 循环中用的. 下面一些例子可以帮助你更好的理解 yield 关键字。下面是摘自 《Programming in Scala》关于 yield 的解释:
For each iteration of your for loop, yield generates a value which will be remembered. It's like the for loop has a buffer you can't see, and for each iteration of your for loop, another item is added to that buffer. When your for loop finishes running, it will return this collection of all the yielded values. The type of the collection that is returned is the same type that you were iterating over, so a Map yields a Map, a List yields a List, and so on.
Also, note that the initial collection is not changed; the for/yield construct creates a new collection according to the algorithm you specify.
上面那段话的意义就是，for 循环中的 yield 会把当前的元素记下来，保存在集合中，循环结束后将返回该集合。Scala 中 for 循环是有返回值的。如果被循环的是 Map，返回的就是 Map，被循环的是 List，返回的就是 List，以此类推。 阅读全文 >>