减少一组空集是否有效? [英] Is it valid to reduce on an empty set of sets?

问题描述

这不应该工作吗?

> val setOfSets = Set[Set[String]]()    
setOfSets: scala.collection.immutable.Set[Set[String]] = Set()

> setOfSets reduce (_ union _)
java.lang.UnsupportedOperationException: empty.reduceLeft
  at scala.collection.TraversableOnce$class.reduceLeft(TraversableOnce.scala:152)
  [...]

推荐答案

Reduce(左右)不能应用于空集合.

Reduce (left and right) cannot be applied on an empty collection.

概念上:

myCollection.reduce(f)

类似于:

myCollection.tail.fold( myCollection.head )( f )

因此集合必须至少有一个元素.

Thus the collection must have at least one element.

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