问题描述
在随后应用诸如map，flatmap等功能时，使用withFilter而不是filter总是表现更好吗?
Is it always more performant to use withFilter instead of filter, when afterwards applying functions like map, flatmap etc.?
为什么仅支持map，flatmap和foreach? (以及预期的功能，如forall/exists)
Why are only map, flatmap and foreach supported? (Expected functions like forall/exists as well)
推荐答案
来自 Scala文档:
因此，filter将采用原始集合并产生一个新集合，但是withFilter将非严格地(即懒惰地)将未过滤的值传递给以后的map/flatMap/withFilter调用，以保存第二次通过(过滤的)集合.因此，当传递到这些后续方法调用时，它将更加高效.
So filter will take the original collection and produce a new collection, but withFilter will non-strictly (i.e. lazily) pass unfiltered values through to later map/flatMap/withFilter calls, saving a second pass through the (filtered) collection. Hence it will be more efficient when passing through to these subsequent method calls.
实际上，withFilter是专门为与这些方法的链一起使用而设计的，因此对于a的理解已被废止了.不需要其他方法(例如forall/exists)，因此尚未将它们添加到withFilter的FilterMonadic返回类型中.
In fact, withFilter is specifically designed for working with chains of these methods, which is what a for comprehension is de-sugared into. No other methods (such as forall/exists) are required for this, so they have not been added to the FilterMonadic return type of withFilter.
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