定义第三方类型类的实例，找不到隐式但显式工作正常

问题描述

我正在使用 Slick的GetResult类型类，并希望使用Shapeless派生GetResult[Option[(A, B, C...)]]

我想要的东西:

给出一个隐式的GetResult[Option[A]], GetResult[Option[B]], ...，
隐式生成GetResult[Option[(A, B, ...)]]

Given an implicit GetResult[Option[A]], GetResult[Option[B]], ...,
implicitly generate a GetResult[Option[(A, B, ...)]]

我尝试过的事情

trait CanGetOption[T] {
    def getOption: GetResult[Option[T]]
}
object CanGetOption {
    // convenience implicit resolver
    def apply[T](implicit canGetOption: CanGetOption[T]): CanGetOption[T] = canGetOption

    // base case: HNil
    implicit val getHNilOption: CanGetOption[HNil] = from(GetResult { _ => Some(HNil) })

    // recursion case: H :: Tail
    implicit def getHConsOption[H, Tail <: HList](
        implicit getHeadOption: GetResult[Option[H]],
        canGetTailOption: CanGetOption[Tail]
    ): CanGetOption[H :: Tail] = from(GetResult[Option[H :: Tail]] { r =>
        val headOpt = getHeadOption(r)
        val tailOpt = canGetTailOption.getOption(r)
        for(head <- headOpt; tail <- tailOpt) yield head :: tail
    })

    // generic case: A, given a A <-> Repr conversion
    // I also tried moving this into a "LowPriorityImplicits" thing, just in case
    implicit def getGenericOption[A, Repr <: HList](
        implicit gen: Generic.Aux[A, Repr],
        getReprOpt: CanGetOption[Repr]
    ): CanGetOption[A] = from(GetResult { r =>
        val reprOpt = getReprOpt.getOption(r)
        reprOpt.map(gen.from)
    })
}

implicit def resolveOptionGetter[T: CanGetOption]: GetResult[Option[T]] =
    CanGetOption[T].getOption

问题:

导入以上内容后，搜索隐式时似乎未考虑resolveOptionGetter:

When I've imported the above, the resolveOptionGetter doesn't seem to be considered when searching for implicits:

scala> implicitly[GetResult[Option[(Int, Int)]]]
<console>:19: error: could not find implicit value for parameter e: scala.slick.jdbc.GetResult[Option[(Int, Int)]]
       implicitly[GetResult[Option[(Int, Int)]]]
             ^

scala> resolveOptionGetter[(Int, Int)]
res1: scala.slick.jdbc.GetResult[Option[(Int, Int)]] = <function1>

为什么编译器在隐式搜索中找不到resolveOptionGetter?我该怎么办?

Why can't the compiler find resolveOptionGetter in the implicit search? What can I do to help it?

推荐答案

问题是slick.jdbc.GetResult是协变的.如果它是不变的，则可以正确地推断类型并可以解决隐式问题.

The thing is that slick.jdbc.GetResult is covariant. If it were invariant, types would be inferred correctly and implicits would be resolved.

一种解决方法是使用自定义不变类型别名GetResult隐藏协变slick.jdbc.GetResult.删除导入slick.jdbc.GetResult并写入您的源文件

A workaround is hiding covariant slick.jdbc.GetResult with custom invariant type alias GetResult. Remove import slick.jdbc.GetResult and write in your source file

type GetResult[T] = slick.jdbc.GetResult[T]

object GetResult {
  def apply[T](implicit f: PositionedResult => T): GetResult[T] = slick.jdbc.GetResult.apply
}

现在implicitly[GetResult[Option[(Int, Int)]]]进行编译.在Scala 2.12.7 + Shapeless 2.3.3 + Slick 3.2.3中进行了测试.

Now implicitly[GetResult[Option[(Int, Int)]]] compiles. Tested in Scala 2.12.7 + Shapeless 2.3.3 + Slick 3.2.3.

差异通常会给隐式解决带来麻烦:

Variance often makes troubles for implicit resolution:

https://github.com/scala/bug/issues/10099

https://github.com/locationtech/geotrellis/issues/1292

具有协方差的隐式分辨率

这篇关于定义第三方类型类的实例，找不到隐式但显式工作正常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！