shapeless-从基础HList派生类型构造函数HList的选择器

Question

I have a Country enum and an HList that is a subset of enum values:

import shapeless._
import iops.hlist.{Comapped, Selector}

sealed trait Country
case object US extends Country
case object DE extends Country
case object CA extends Country
...

val countries = US :: DE :: HNil

I have a Price class and PriceTable as below:

case class Price[C <: Country](value: Double)

class PricesTable[CountryList <: HList, PriceList <: HList](prices: PriceList)
  (implicit comapped: Comapped.Aux[PriceList, Price, CountryList]) {

def priceFor[C <: Country](implicit selector: Selector[CountryList, C]: Price[C] = 
  prices.select[Price[C]]
} 

val pricesTable = new PricesTable(Price[US.type](20) :: Price[DE.type](25) :: HNil)

The priceFor statement does not compile since a Selector[PriceList, Price[C]] is not in scope.

The code calling priceFor only has access to a Selector[CountryList, C] but not a Selector[PriceList, Price[C]] given that CountryList =:= countries.type .

Is there a way to derive a Selector[PriceList, Price[C]] from a Selector[CountryList, C] given that Comapped.Aux[PriceList, Price, CountryList] proves the relationship?

Answer 1

If what you want to achieve is to get the price of a given country type, since Country is a type parameter of Price , how about:

class PricesTable[PriceList <: HList](prices: PriceList)(implicit lubC: LUBConstraint[PriceList, Price[_]]) {
  def priceFor[C <: Country](implicit selector: Selector[PriceList, Price[C]]): Price[C] =
    prices.select[Price[C]]
}