什么是“提升”在斯卡拉？

本文介绍了什么是“提升”在斯卡拉？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 29岁程序员，3月因学历无情被辞！ 有时，当我阅读Scala生态系统中的文章时，我读到了提升/提升的术语。不幸的是，它没有解释到底是什么意思。我做了一些研究，似乎解除与功能值或类似的东西有关，但我无法找到一个文本解释以新手友好的方式实际提升的内容。 电梯框架，它的名字取消了，但并没有帮助回答这个问题。 什么是Scala中的提升？ 解决方案有几个用法： $ b PartialFunction 记住 PartialFunction [A，B] 是为域 A （由 isDefinedAt 方法指定）。您可以将 PartialFunction [A，B] 提升为函数[A，Option [B]] 。也就是定义在 A 的整个上的一个函数，但其​​值是 Option [B] $ b 这是通过显式调用方法 lift 来完成的， PartialFunction 。 scala> val pf：PartialFunction [Int，Boolean] = {case i if i> 0 => i％2 == 0} pf：PartialFunction [Int，Boolean] =< function1> scala> pf.lift res1：Int => Option [Boolean] =< function1> scala> res1（-1） res2：Option [Boolean] =无 scala> res1（1） res3：Option [Boolean] =一些（false） 方法 您可以将方法调用提升为一个函数。这叫做 eta-expansion （感谢Ben James）。举例来说： scala> def times2（i：Int）= i * 2 times2：（i：Int）Int 我们通过应用下划线将一个方法提升为一个函数 scala> val f = times2 _ f：Int => Int =< function1> scala> f（4） res0：int = 8 请注意方法和函数之间的根本区别。 （函数）类型是一个实例（即它是一个值）。 （Int => Int） 函子 函数（由 scalaz 定义）是某个容器（我使用非常宽松的）， F 如果我们有一个 F [A] 和一个函数 A => B ，那么我们可以得到一个 F [B] （想想，例如， F = List 和 map 方法） 我们可以如下编码这个属性： trait Functor [F [_]] { def map [A​​，B]（fa：F [A]）（ f：A => B）：F [B] } 这是同构的能够提升功能 A => B 放入函子的域中。那就是： $ $ $ $ $ $ $ c $ def def [F [_]：Functor，A，B]（f：A => B） ：F [A] => F [B] 也就是说，如果 F 是一个仿函数，我们有一个函数 A => B ，我们有一个函数 F [A] => F [B] 。您可以尝试并实施 lift 方法 - 这非常简单。 Monad变形金刚 hcoopz 在下面说（我刚刚意识到这样可以节省大量的不必要的代码），术语lift也有意义在 Monad变形金刚中。回想一下，monad变形金刚是一种叠加monad的方式（单体不合成）。 例如，假设你有一个函数，它返回一个 IO [Stream [A]] 。这可以转换为monad转换器 StreamT [IO，A] 。现在你可能希望提升一些其他的值，也许它也是一个 StreamT > IO [B] 。你可以这样写： StreamT.fromStream（iob map（b => Stream（b））） 或者： iob.liftM [StreamT] 这引出了一个问题：为什么我要将 IO [B] 转换为 StreamT [IO，B] ？。答案将是利用组合的可能性。假设你有一个函数 f：（A，B）=> C lazy val f：（A，B）=> C = ??? val cs = 用于{a b< - bs.liftM [StreamT] // bs只是一个IO [B] } 产生f（a，b） cs.toStream //是一个Stream [IO [C]]，cs是一个StreamT [IO ，C] Sometimes when I read articles in the Scala ecosystem I read the term "lifting" / "lifted". Unfortunately, it is not explained what that exactly means. I did some research, and it seems that lifting has something to do with functional values or something like that, but I was not able to find a text that explains what lifting actually is about in a beginner friendly way.There is additional confusion through the Lift framework which has lifting in its name, but it doesn't help answer the question.What is "lifting" in Scala? 解决方案 There are a few usages:PartialFunctionRemember a PartialFunction[A, B] is a function defined for some subset of the domain A (as specified by the isDefinedAt method). You can "lift" a PartialFunction[A, B] into a Function[A, Option[B]]. That is, a function defined over the whole of A but whose values are of type Option[B]This is done by the explicit invocation of the method lift on PartialFunction.scala> val pf: PartialFunction[Int, Boolean] = { case i if i > 0 => i % 2 == 0}pf: PartialFunction[Int,Boolean] = <function1>scala> pf.liftres1: Int => Option[Boolean] = <function1>scala> res1(-1)res2: Option[Boolean] = Nonescala> res1(1)res3: Option[Boolean] = Some(false)MethodsYou can "lift" a method invocation into a function. This is called eta-expansion (thanks to Ben James for this). So for example:scala> def times2(i: Int) = i * 2times2: (i: Int)IntWe lift a method into a function by applying the underscorescala> val f = times2 _f: Int => Int = <function1>scala> f(4)res0: Int = 8Note the fundamental difference between methods and functions. res0 is an instance (i.e. it is a value) of the (function) type (Int => Int)FunctorsA functor (as defined by scalaz) is some "container" (I use the term extremely loosely), F such that, if we have an F[A] and a function A => B, then we can get our hands on an F[B] (think, for example, F = List and the map method)We can encode this property as follows:trait Functor[F[_]] { def map[A, B](fa: F[A])(f: A => B): F[B]}This is isomorphic to being able to "lift" the function A => B into the domain of the functor. That is:def lift[F[_]: Functor, A, B](f: A => B): F[A] => F[B]That is, if F is a functor, and we have a function A => B, we have a function F[A] => F[B]. You might try and implement the lift method - it's pretty trivial.Monad TransformersAs hcoopz says below (and I've just realized that this would have saved me from writing a ton of unnecessary code), the term "lift" also has a meaning within Monad Transformers. Recall that a monad transformers are a way of "stacking" monads on top of each other (monads do not compose).So for example, suppose you have a function which returns an IO[Stream[A]]. This can be converted to the monad transformer StreamT[IO, A]. Now you may wish to "lift" some other value an IO[B] perhaps to that it is also a StreamT. You could either write this:StreamT.fromStream(iob map (b => Stream(b)))Or this:iob.liftM[StreamT]this begs the question: why do I want to convert an IO[B] into a StreamT[IO, B]?. The answer would be "to take advantage of composition possibilities". Let's say you have a function f: (A, B) => Clazy val f: (A, B) => C = ???val cs = for { a <- as //as is a StreamT[IO, A] b <- bs.liftM[StreamT] //bs was just an IO[B] } yield f(a, b)cs.toStream //is a Stream[IO[C]], cs was a StreamT[IO, C] 这篇关于什么是“提升”在斯卡拉？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！