查找 Shapeless HList 的类型类实例 [英] Find type class instances for Shapeless HList

问题描述

说我有一个特征 Show[T]，比如 Scalaz 中的特征:https://github.com/scalaz/scalaz/blob/scalaz-seven/core/src/main/scala/scalaz/Show.scala#L9

Say that I have a trait Show[T] such as the one in Scalaz: https://github.com/scalaz/scalaz/blob/scalaz-seven/core/src/main/scala/scalaz/Show.scala#L9

我还有一个 Shapeless HList，它看起来像 "1" :: 2 :: 3L :: HNil.

I also have a Shapeless HList that may look like "1" :: 2 :: 3L :: HNil.

有没有办法找到每个元素的 Show 实例并应用 shows 这样我最终得到 "1" :: "2" :: "3L" :: HNil?

Is there a way to find the Show instance for each element and apply shows such that I end up with "1" :: "2" :: "3L" :: HNil?

如果任何元素属于在范围内没有隐式 Show 实例的类型，我会想要一个编译错误.

If any element were of a type that did not have an implicit Show instance in scope I would want a compile error.

我认为，如果我建立一个 Show 实例的 HList，我应该能够使用 zipApply 来获取 HList 我想要，但我不知道是否有办法让 Scala 推断 Show 实例的 HList 而不是我通过手.

I think that if I build up an HList of the Show instances I should be able to use zipApply to get the HList I want, but I don't know if there is a way to get have Scala infer the HList of Show instances instead of me building it up by hand.

推荐答案

如果您的目标是应用 Show 实例并且您不关心构建 HList，最简单的方法可能是使用多态函数:

If your goal is to apply the Show instances and you don't otherwise care about building up an HList of them, the easiest approach is probably to use a polymorphic function:

import scalaz._, Scalaz._, shapeless._

val xs = "1" :: 2 :: 3L :: HNil

object show extends Poly1 {
  implicit def forShowable[A: Show] = at[A](_.shows)
}

val strings: String :: String :: String :: HNil = xs map show

您可以通过稍微更改 Poly1 来获得实例的 HList:

You could get an HList of the instances by changing the Poly1 a bit:

object showInstance extends Poly1 {
  implicit def forShowable[A: Show] = at[A](_ => Show[A])
}

在某些情况下，定义您自己的类型类以收集您拥有特定类型类实例的证据会很有用:

In some cases it can be useful to define your own type class to collect evidence that you've got certain type class instances:

trait AllShowable[L <: HList, S <: HList] {
  def instances: S
}

implicit object hnilAllShowable extends AllShowable[HNil, HNil] {
  def instances = HNil
}

implicit def hlistAllShowable[H: Show, TL <: HList, TS <: HList](
  implicit ts: AllShowable[TL, TS]
) = new AllShowable[H :: TL, Show[H] :: TS] {
  def instances = Show[H] :: ts.instances
}

但通常使用需要实例的多态函数映射会工作得很好.

But usually mapping with a polymorphic function that requires the instances will work just fine.

这篇关于查找 Shapeless HList 的类型类实例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IT屋！