无法映射到 HList [英] Unable to map on HList

问题描述

我试图用无形的方式解决这个问题.但是由于某种原因我无法映射 HList.我会让代码不言自明.

I was trying to solve this problem with shapeless. However I am for some reason unable to map on the HList. I'll let the code speak for itself.

import shapeless._
import HList._

case class Foo(a: Option[Int], b: Option[Int])

val a = Foo(Some(3), None)

val b = Foo(Some(22), Some(1))

implicit val fooIso = HListIso(Foo.apply _, Foo.unapply _)

val mapper = new (({ type O2[+A] = (Option[A], Option[A]) })#O2 ~> Option) {
  def apply[A](x: (Option[A], Option[A])): Option[A] = x._1.orElse(x._2)
}

fooIso.fromHList(fooIso.toHList(a).zip(fooIso.toHList(b)).map(mapper))

错误信息是:

<console>:55: error: could not find implicit value for parameter mapper: shapeless.Mapper[java.lang.Object with shapeless.~>[[+A](Option[A], Option[A]),Option],shapeless.::[(Option[Int], Option[Int]),shapeless.::[(Option[Int], Option[Int]),shapeless.HNil]]]
              fooIso.fromHList(fooIso.toHList(a).zip(fooIso.toHList(b)).map(mapper))
                                                                           ^

为什么映射不起作用?

推荐答案

有一个简单的解决方法:只需将函数定义为 object 而不是 val:

There's an easy fix: just define your function as an object instead of a val:

object f extends (({ type O2[+A] = (Option[A], Option[A]) })#O2 ~> Option) {
  def apply[A](x: (Option[A], Option[A])): Option[A] = x._1 orElse x._2
}

(注意，我将函数命名为 f 而不是 mapper 以避免与 mapper 隐式参数混淆到 map.)

(Note that I've named the function f instead of mapper to avoid confusion with the mapper implicit argument to map.)

我不确定我能帮助解决为什么——在某些时候我试图弄清楚为什么 val 对这种事情不起作用的细节在 Shapeless 中，我不记得我走了多远.

I'm not sure I can help with why—at some point I tried to work out the details of why val wouldn't work for this kind of thing in Shapeless, and I don't remember how far I got.

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