为什么 PartialFunction 上没有接受总计函数的 orElse 方法？

发布于 2024-12-09 14:48:46 字数 190 浏览 0 评论 0原文

为什么 PartialFunction[A, B] 类中没有具有以下签名的方法？

def orElse[A1 <: A, B1 >: B](that: A1 => B1): A1 => B1

缺乏这种方法背后是否有某种逻辑原因，或者仅仅是一个疏忽？

原文

Why is there no method with following signature on class PartialFunction[A, B]?

def orElse[A1 <: A, B1 >: B](that: A1 => B1): A1 => B1

Is there some logical reason behind the absence of this method, or was it a mere oversight?

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|煩躁 2024-12-16 14:48:46

因为通过提升部分功能来实现相同的效果是微不足道的
partialFunc.lift(arg) getOrElse (totalFunc(arg))
因为 Scala 通常会尝试避免重载
因为没有人想到添加它，到目前为止可能还不需要它
因为添加到标准库中的每个方法都会产生不断增长的成本下游维护方面

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最单纯的乌龟 2024-12-16 14:48:46

考虑一下，

scala> object O {
     |   def f(g: Int => Int) = g(1)
     |   def f(g: PartialFunction[Int, Int]) = g(2).toString
     | }
defined module O

scala> O f { _ * 1 }
res3: Int = 1

那么，现在如何链接部分函数呢？或者，换句话说，如果您描述的重载在库中，并且我写了这样的内容：

type PF = PartialFunction[Any, Int]
val pf1: PF = { case n: Int => n }
val pf2: PF = pf1 orElse { case x: String => x.length }
val pf3: PF = pf2 orElse { case d: Double => d.toInt }

由于类型不明确，我会在 pf2 上收到一条错误消息。相反，如果我写：

val pf2 = pf1 orElse ((_: Any) match { case x: String => x.length })
val pf3 = pf2 orElse ((_: Any) match { case d: Double => d.toInt })

那么我在 pf3 上收到错误，因为 pf2 将是 Function1。

Consider,

scala> object O {
     |   def f(g: Int => Int) = g(1)
     |   def f(g: PartialFunction[Int, Int]) = g(2).toString
     | }
defined module O

scala> O f { _ * 1 }
res3: Int = 1

So, how do you chain partial functions now? Or, in other words, if the overload you describe was in the library, and I wrote this:

type PF = PartialFunction[Any, Int]
val pf1: PF = { case n: Int => n }
val pf2: PF = pf1 orElse { case x: String => x.length }
val pf3: PF = pf2 orElse { case d: Double => d.toInt }

I'd get an error message on pf2 because of the ambiguity of the type. If, instead, I write:

val pf2 = pf1 orElse ((_: Any) match { case x: String => x.length })
val pf3 = pf2 orElse ((_: Any) match { case d: Double => d.toInt })

Then I get an error on pf3, because pf2 will be a Function1.

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淡写薰衣草的香 2024-12-16 14:48:46

看来标准库中没有这些函数没有任何充分的理由，除了监督之外，或者也许没有人像在标准库中那样频繁地需要这些函数。

我已经定义了从 A => 的隐式转换； B 到 PartialFunction[A, B] 似乎可以处理这种情况和其他类似的情况，并且不会导致不利影响。

scala> implicit def fToPf[A, B](f: A => B) = new PartialFunction[A, B] {
     |   def isDefinedAt(a: A) = true
     |   def apply(a: A) = f(a)
     | }
fToPf: [A, B](f: A => B)java.lang.Object with PartialFunction[A,B]

It appears there isn't any good reason for the absence of these functions in the standard library, other than oversight or maybe nobody needs these as often as to have them in standard library.

I have defined an implicit conversion from A => B to PartialFunction[A, B] which seems to take care of this and other similar cases, and doesn't lead to adverse effects.

scala> implicit def fToPf[A, B](f: A => B) = new PartialFunction[A, B] {
     |   def isDefinedAt(a: A) = true
     |   def apply(a: A) = f(a)
     | }
fToPf: [A, B](f: A => B)java.lang.Object with PartialFunction[A,B]

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