为什么 Play 2.0.2 中有 Form 的 fold 方法?

问题描述

我指的是:

http://www.playframework.org/documentation/api/2.0.2/scala/index.html#play.api.data.Form

如果您搜索名为 fold 的方法，它会显示用于处理表单的方法.这种方法被称为折叠有什么原因吗?鉴于 fold 已经有 list like objects 的含义，这个名字似乎很容易引起混淆.

If you search for a method called fold, it shows a method used for handling the form. Is there a reason why this method is called fold? Given that fold already has a meaning for list like objects, it seems that this name could easily cause confusion.

推荐答案

Form 上的 fold 与表单上的 fold 非常接近Scala 标准库中的 Either 类，它同样经常用于捕获可能成功的过程的结果(在这种情况下，您有一个包含结果的 Right)或失败(在这种情况下，您有一个 Left 包含错误，或者可能是剩余的输入等).因此，我将在这里使用 Either 作为示例.如有必要，只需将 Form[T] 想象成一种 Either[Form[T], T].

The fold on Form is pretty close to the fold on the Either class in the Scala standard library, which is similarly often used to capture the outcome of a process that could either succeed (in which case you have a Right containing the result) or fail (in which case you have a Left containing an error, or maybe leftover input, etc.). So I'll use Either as an example here. Just picture Form[T] as a kind of Either[Form[T], T] if necessary.

我们可以(非常非正式地)将列表想象成具有许多不同的形状"(空列表、长度为一的列表、长度为二的列表等)，并且 fold(或 foldLeft，在下面的例子中)作为一种方法，将任何正确类型的列表折叠为单一类型的事物，无论其形状如何:

We can (very informally) imagine lists as having lots of different "shapes" (empty lists, lists of length one, length two, etc.), and fold (or foldLeft, in the following example) as a method that collapses any list of the proper type to a single kind of thing, no matter what its shape is:

scala> def catInts(xs: List[Int]): String = xs.foldLeft("")(_ + _.toString)
catInts: (xs: List[Int])String

scala> catInts(List(1, 2, 3, 4))
res0: String = 1234

scala> catInts(Nil)
res1: String = ""

在任一/表单上折叠

同样我们可以把Either想象成有两个形状(Right 和Left)，它的fold 为一种方法，它采用任一形状的 Either 并返回一种事物.假设我们有以下方法将字符串解析为整数并返回 Either:

Fold on Either / Form

Similarly we can imagine Either as having two shapes (Right and Left), and its fold as a method that takes an Either of either shape and returns a single kind of thing. Say we have the following method for parsing strings as integers and returning an Either:

def parseInt(s: String): Either[String, Int] =
  try Right(s.toInt) catch {
    case _: NumberFormatException => Left(s)
  }

以及以下使用fold折叠Either的方法:

And the following method that uses fold to collapse the Either:

def toDefault(e: Either[String, Int]): Int = e.fold(_ => 42, identity)

我们可以这样使用:

scala> toDefault(parseInt("AAARGH!!"))
res2: Int = 42

scala> toDefault(parseInt("123"))
res3: Int = 123

这显然是非常印象派和手工波浪式的，但它可能有助于更直观地了解不同的 fold 方法如何从根本上说是同一种东西.你可以看到我在上面评论中链接的问题或有关 catamorphisms 的维基百科条目 了解更多详情.

This is all obviously very impressionistic and hand-wavy, but it might help give a more intuitive sense of how the different fold methods are fundamentally the same kind of thing. You can see the question I linked in a comment above or the Wikipedia entry on catamorphisms for more detail.

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