为什么在Option [T]的HList上映射不起作用? [英] Why does mapping over an HList of Option[T] not work?

问题描述

这不能编译，我也不明白为什么:

This does not compile and I do not understand why:

import shapeless._
import poly._

object option extends (Option ~> List) {
    def apply[T](t: Option[T]) = t.toList
}

val simple = Some(1) :: Some("hello") :: Some(true) :: HNil
val opts: List[Int] :: List[String] :: List[Boolean] :: HNil = simple map option

这有点令人惊讶，因为它可以编译:

This is somewhat surprising, because this does compile:

import shapeless._
import poly._

object choose extends (Set ~> Option) {
    def apply[T](s: Set[T]) = s.headOption
}

val sets = Set(1) :: Set("foo") :: HNil
val opts: Option[Int] :: Option[String] :: HNil = sets map choose

推荐答案

问题是您使用的是Some而不是Option.编译器抱怨说，尽管正在使用Option映射器，但找不到任何东西可以转换Some的列表.要解决此问题，您可以:

The problem is your usage of Some instead of Option. The compiler complains that it can't find anything to convert a list of Somes though your Option mapper is being used. To fix this you can:

将您的简单列表结构更改为使用Option而不是Some:

Change your simple list construction to use Option instead of Some:

  object option extends (Option ~> List) {
    def apply[T](t: Option[T]) = t.toList
  }

  val simple = Option(1) :: Option("hello") :: Option(true) :: HNil
  val opts2: List[Int] :: List[String] :: List[Boolean] :: HNil = simple map option

使用Some作为映射器的类型:

Use Some as the type of your mapper:

  object option extends (Some ~> List) {
    def apply[T](t: Some[T]) = t.toList
  }

  val simple = Some(1) :: Some("hello") :: Some(true) :: HNil
  val opts2: List[Int] :: List[String] :: List[Boolean] :: HNil = simple map option

或强制将简单类型下调为Option:

Or force the type of simple to be downcast to Option:

  object option extends (Option ~> List) {
    def apply[T](t: Option[T]) = t.toList
  }

  val simple: Option[Int] :: Option[String] :: Option[Boolean] :: HNil = Some(1) :: Some("hello") :: Some(true) :: HNil
  val opts2: List[Int] :: List[String] :: List[Boolean] :: HNil = simple map option

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