问题描述
我几乎完全从此处找到的派生展示"示例中获取了以下代码:https://github.com/milessabin/kittens
I have taken the following code almost entirely from the 'derive show' example found here: https://github.com/milessabin/kittens
import cats._, cats.derived._, cats.implicits._
object Test extends App {
case class Address(street: String, city: String, state: String)
case class ContactInfo(phoneNumber: String, address: Address)
case class People(name: String, age: Double, contactInfo: ContactInfo)
val mike = People("Mike", 1.23, ContactInfo("202-295-3928", Address("1 Main ST", "Chicago", "IL")))
implicit val addressShow: Show[Address] = new Show[Address] {
def show(a: Address) = s"${a.street}, ${a.city}, ${a.state}"
}
// I would like to use a custom version of Show for Doubles...
// implicit val doubleShow: Show[Double] = new Show[Double] {
// def show(d: Double) = s"Custom Double Show: $d"
// }
implicit val peopleShow: Show[People] = {
import auto.show._
semi.show
}
println(mike.show)
}
我希望能够使用 Show[Double] 的自定义版本(我不是真的,但这是一个很好的最小示例,它说明了我实际遇到的问题)
I would like to be able to use a custom version of Show[Double] (I don't really, but this is a nice minimal example which illustrate the problem I'm actually having)
如果我取消注释我的 doubleShow
,我会收到以下错误:
If I un-comment my doubleShow
I get the following error:
Error:(25, 10) ambiguous implicit values:
both value emptyProductDerivedShow in trait MkShowDerivation of type => cats.derived.MkShow[shapeless.HNil]
and method emptyCoproductDerivedShow in trait MkShowDerivation of type => cats.derived.MkShow[shapeless.CNil]
match expected type cats.derived.MkShow[A]
semi.show
我怎样才能让它工作?我本来希望我的本地定义的隐式胜过任何从猫进口的......
How can I get this working? I would have expected my locally defined implicits to win out over any imported from cats...
推荐答案
尝试使用隐式对象代替隐式 val
Try implicit object instead of implicit val
implicit object doubleShow extends Show[Double] {
def show(d: Double) = s"Custom Double Show: $d"
}
或使用隐式 val 并更改导入(不要 importcats.instances.double._
)
or use implicit val and change imports (don't import cats.instances.double._
)
import cats._, cats.derived._
import cats.syntax.show._
import cats.instances.string._ // People uses String and Double
implicit val doubleShow: Show[Double] = new Show[Double] {
def show(d: Double) = s"Custom Double Show: $d"
}
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