Scala的理解力何时会变得懒惰? [英] When are scala's for-comprehensions lazy?

问题描述

在Python中，我可以执行以下操作:

In Python, I can do something like this:

lazy = ((i,j) for i in range(0,10000) for j in range(0,10000))
sum((1 for i in lazy))

需要一段时间，但是内存使用量是恒定的.

It will take a while, but the memory use is constant.

scala中的相同构造:

The same construct in scala:

(for(i<-0 to 10000; j<-i+1 to 10000) yield (i,j)).count((a:(Int,Int)) => true)

过一会儿，我得到一个java.lang.OutOfMemoryError，即使应该懒惰地对其求值.

After a while, I get a java.lang.OutOfMemoryError, even though it should be evaluated lazily.

推荐答案

Scala的理解力并没有天生地懒惰.它是语法糖*，不会改变您两个范围的组合会急切的事实.

Nothing's inherently lazy about Scala's for-comprehension; it's syntactic sugar* which won't change the fact that the combination of your two ranges will be eager.

如果您使用范围的惰性view，则理解的结果也将是惰性的:

If you work with lazy views of your ranges, the result of the comprehension will be lazy too:

scala> for(i<-(0 to 10000).view; j<-(i+1 to 10000).view) yield (i,j)
res0: scala.collection.SeqView[(Int, Int),Seq[_]] = SeqViewN(...)

scala> res0.count((a: (Int, Int)) => true)
res1: Int = 50005000

这里的惰性与理解力无关，但是因为在某些类型的容器上调用flatMap或map(请参见下文)时，您会在相同类型的容器中返回结果.因此，理解力只会保留您所放内容的惰性(或缺乏).

The laziness here is nothing to do with the for-comprehension, but because when flatMap or map (see below) are called on some type of container, you get back a result in the same type of container. So, the for-comprehension will just preserve the laziness (or lack of) of whatever you put in.

*用于类似的内容:

(0 to 10000).flatMap(i => (i+1 to 10000).map(j => (i, j)))

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