fold和foldLeft或foldRight之间的区别? [英] Difference between fold and foldLeft or foldRight?

问题描述

注意:我使用的是Scala 2.8，这可能是个问题吗?

NOTE: I am on Scala 2.8—can that be a problem?

为什么不能使用与foldLeft或foldRight相同的方式使用fold函数?

Why can't I use the fold function the same way as foldLeft or foldRight?

在设置scaladoc 中表示:

折叠的结果只能是此并行集合的类型参数T的超类型.

但是我在函数签名中没有看到类型参数T:

But I see no type parameter T in the function signature:

def fold [A1 >: A] (z: A1)(op: (A1, A1) ⇒ A1): A1

foldLeft-Right和fold有什么区别，如何使用后者?

What is the difference between the foldLeft-Right and fold, and how do I use the latter?

例如，我将如何编写折叠以将所有元素添加到列表中?对于foldLeft，它将是:

For example how would I write a fold to add all elements in a list? With foldLeft it would be:

val foo = List(1, 2, 3)
foo.foldLeft(0)(_ + _)

// now try fold:
foo.fold(0)(_ + _)
>:7: error: value fold is not a member of List[Int]
  foo.fold(0)(_ + _)
    ^

推荐答案

您认为Scala的旧版本存在问题是正确的.如果您查看 scaladoc页面对于Scala 2.8.1，您不会在其中看到任何折叠定义(这与您的错误消息一致).显然，fold是在Scala 2.9中引入的.

You're right about the old version of Scala being a problem. If you look at the scaladoc page for Scala 2.8.1, you'll see no fold defined there (which is consistent with your error message). Apparently, fold was introduced in Scala 2.9.

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