有没有一种方法可以通过单个HList定义多个隐式证据? [英] Is there a way to define multiple implicit evidences via a single HList?

问题描述

我有一段代码，在概念上类似于以下代码:

I have a piece of code, conceptually similar to the following one:

//library code
trait Support[K, V]

def partialHandler[K, V](key: K, value: V)(implicit ev: Support[K, V]) = ???

//user code
implicit val intIntSupport = new Support[Int, Int] {}
implicit val intStringSupport = new Support[Int, String] {}
...

partialHandler(1, "foo)
partialHandler(1, 1)

我想知道是否有一种方法可以让该库的用户更优雅地定义受支持的(K, V)类型，例如:

I wonder if there is a way to let users of this library define supported (K, V) types more elegantly, e.g.:

val supportedTypes = new Support[Int, Int] {} :: new Support[Int, String] {} :: HNil

(本质上，我正在寻找从几乎未知的HList到Support[K, V]的隐式转换.这看起来不可行，但也许我遗漏了一些东西.)

(In essence, I'm looking for an implicit conversion from pretty much unknown HList to Support[K, V]. This doesn't look doable, but maybe I'm missing something.)

推荐答案

尝试使supportedTypes隐式

import shapeless.ops.hlist.Selector
import shapeless.{HList, HNil}

// library code
trait Support[K, V]

object Support {
  implicit def mkSupport[L <: HList, K, V](implicit l: L, sel: Selector[L, Support[K, V]]): Support[K, V] = null
}

def partialHandler[K, V](key: K, value: V)(implicit ev: Support[K, V]) = ???

//user code
implicit val supportedTypes = new Support[Int, Int] {} :: new Support[Int, String] {} :: new Support[Long, Double] {} :: HNil

partialHandler(1, "foo")
partialHandler(1, 1)
partialHandler(1L, 1.0)
// partialHandler("foo", "bar") // doesn't compile

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