foldRight在无限的惰性结构上 [英] foldRight on infinite lazy structure

问题描述

根据 http://en.wikipedia.org/wiki/Fold_(higher-order_function )，如果不需要评估完整列表，则可以对无限列表进行右折操作.这可以在haskell的操作中看到:

According to http://en.wikipedia.org/wiki/Fold_(higher-order_function), a right fold can operate on infinite lists if the full list does not have to be evaluated. This can be seen in action in haskell:

Prelude> take 5 (foldr (:) [] [1 ..])
[1,2,3,4,5]

对于流来说，这似乎在scala中效果不佳:

This does not seem to work well in scala for streams:

Stream.from(1).foldRight(Stream.empty[Int])( (i, s) => i #:: s).take(5)
// StackOverflowError

或在迭代器上:

Iterator.from(1).foldRight(Iterator.empty: Iterator[Int]){ (i, it) => 
  Iterator.single(i) ++ it
}.take(5)
// OutOfMemoryError: Java heap space

在Scala中是否有可行的解决方案来实现懒惰折叠?

Is there a practical solution to achieve a lazy fold right in Scala?

推荐答案

This article makes the same observation, and suggests a lazy solution using scalaz. Credit to the author, and Tony Morris.

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