正确的投影混淆（理解脱糖）

本文介绍了正确的投影混淆（理解脱糖）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！问题描述我可以在scala for-comprehension中使用 = （如 6.19 的SLS ）如下所示： h2> 假设我有一些函数 String =>选项[Int] ： scala> def intOpt（s：String）= try {Some（s.toInt）} catch {case _ =>无} intOpt：（s：String）选项[Int] 然后我可以使用它因此 scala>为{ | str< - Option（1） |我< - intOpt（str） | val j = i + 10 //注意在生成器 |中使用= } |产量j res18：Option [Int] =一些（11）这是我的理解这基本上相当于： scala>选项（1）flatMap {str => intOpt（str）} map {i => i + 10} map {j => j} res19：Option [Int] =一些（11）也就是说，嵌入式生成器是一种将 map 注入到 flatMap 调用序列中的方法。到目前为止这么好。或者.RightProjection 我真正想做的事：使用与前面使用任一 monad 的示例类似的理解。 Either.RightProjection monad / functor，它不起作用： scala> def intEither（s：String）：或者[Throwable，Int] = |尝试{Right（s.toInt）} catch {case x => Left（x）} intEither：（s：String）[Throwable，Int] 然后使用： scala>为{ | str< - Option（1）。toRight（new Throwable（））right |我< - intEither（str）.right //注意右投影使用 | val j = i + 10 | } | （java.lang.Throwable，（Int，Int）] i< - intEither（str）;返回值为j < console>：17：error：value map不是Product的一个成员，）.right ^ 这个问题与这个函数有关，投影期望作为其 flatMap 方法的一个参数（即它期望一个 R => [L，R] ）。但修改为不在第二个生成器上调用 right ，它仍然不会编译。阶>为{ | str< - Option（1）。toRight（new Throwable（））right |我< - intEither（str）//否rightprojection | val j = i + 10 | } | yield j < console>：17：error：value map不是Either [Throwable，Int] i ^ 的成员。 code> 超级混乱但现在我倍感困惑。下面的例子很好： scala>为{ | x< - 右[Throwable，String]（1）。右 | y< - 右[Throwable，String]（x）.right //注意这里的right | } yield y.toInt res39：或者[Throwable，Int] = Right（1）但是这不是： scala> Right [Throwable，String]（1）。right flatMap {x =>右[Throwable，String]（x）.right} map {y => y.toInt} < console>：14：error：type mismatch; found：Either.RightProjection [Throwable，String] required：[？，？] Right [Throwable，String]（1）。right flatMap {x =>右[Throwable，String]（x）.right} map {y => y.toInt} ^ 我认为这些是相同的发生了什么事情？如何嵌入 = 在> ？解析方案 div> 不能将 = 嵌入到for-comprehension中的事实与这个问题由Jason Zaugg报道;解决的办法是右偏或者（或者创建一个与它同构的新数据类型）。巨大的困惑，你不正确地扩大了糖。（b c $ b ）} yield z（c）是 x（a）flatMap {b => y（b）map {c => z（c）}} 而不是 x（a）flatMap {b => y（b）} map {c => z（c）} 因此您应该这样做： scala> Right [Throwable，String]（1）。right flatMap {x =>右[Throwable，String]（x）.right map {y => y.toInt}} res49：[Throwable，Int] = Right（1）关于desugaring（`j = i + 10`问题）的更多乐趣对于{b c x1 = f1（b） x2 = f2（b，x1） ... xn = fn （.....）d }得到w（d）被解析成 x（a）flatMap {b => y（b）map {c => x1 = .. ... xn = .. （c，x1，..，xn）} flatMap {（_c1，_x1，。。，_xn）=> z（_c1，_xn）map w}} $ c> y（b）的结果类型为或者不包含 map 定义。 I can use an = in a scala for-comprehension (as specified in section 6.19 of the SLS) as follows:OptionSuppose I have some function String => Option[Int]:scala> def intOpt(s: String) = try { Some(s.toInt) } catch { case _ => None }intOpt: (s: String)Option[Int]Then I can use it thusscala> for { | str <- Option("1") | i <- intOpt(str) | val j = i + 10 //Note use of = in generator | } | yield jres18: Option[Int] = Some(11)It was my understanding that this was essentially equivalent to:scala> Option("1") flatMap { str => intOpt(str) } map { i => i + 10 } map { j => j }res19: Option[Int] = Some(11)That is, the embedded generator was a way of injecting a map into a sequence of flatMap calls. So far so good.Either.RightProjectionWhat I actually want to do: use a similar for-comprehension as the previous example using the Either monad.However, if we use it in a similar chain, but this time using the Either.RightProjection monad/functor, it doesn't work:scala> def intEither(s: String): Either[Throwable, Int] = | try { Right(s.toInt) } catch { case x => Left(x) }intEither: (s: String)Either[Throwable,Int]Then use:scala> for { | str <- Option("1").toRight(new Throwable()).right | i <- intEither(str).right //note the "right" projection is used | val j = i + 10 | } | yield j<console>:17: error: value map is not a member of Product with Serializable with Either[java.lang.Throwable,(Int, Int)] i <- intEither(str).right ^The issue has something to do with the function that a right-projection expects as an argument to its flatMap method (i.e. it expects an R => Either[L, R]). But modifying to not call right on the second generator, it still won't compile.scala> for { | str <- Option("1").toRight(new Throwable()).right | i <- intEither(str) // no "right" projection | val j = i + 10 | } | yield j<console>:17: error: value map is not a member of Either[Throwable,Int] i <- intEither(str) ^Mega-ConfusionBut now I get doubly confused. The following works just fine:scala> for { | x <- Right[Throwable, String]("1").right | y <- Right[Throwable, String](x).right //note the "right" here | } yield y.toIntres39: Either[Throwable,Int] = Right(1)But this does not:scala> Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right } map { y => y.toInt }<console>:14: error: type mismatch; found : Either.RightProjection[Throwable,String] required: Either[?,?] Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right } map { y => y.toInt } ^I thought these were equivalentWhat is going on?How can I embed an = generator in a for comprehension across an Either? 解决方案 The fact that you cannot embed the = in the for-comprehension is related to this issue reported by Jason Zaugg; the solution is to Right-bias Either (or create a new data type isomorphic to it).For your mega-confusion, you expanded the for sugar incorrectly. The desugaring offor { b <- x(a) c <- y(b)} yield z(c)isx(a) flatMap { b => y(b) map { c => z(c) }} and notx(a) flatMap { b => y(b)} map { c => z(c) }Hence you should have done this:scala> Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right map { y => y.toInt } }res49: Either[Throwable,Int] = Right(1)More fun about desugaring (the `j = i + 10` issue)for { b <- x(a) c <- y(b) x1 = f1(b) x2 = f2(b, x1) ... xn = fn(.....) d <- z(c, xn)} yield w(d)is desugared intox(a) flatMap { b => y(b) map { c => x1 = .. ... xn = .. (c, x1, .., xn) } flatMap { (_c1, _x1, .., _xn) => z(_c1, _xn) map w }}So in your case, y(b) has result type Either which doesn't have map defined. 这篇关于正确的投影混淆（理解脱糖）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！

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正确的投影混淆（理解脱糖）