scala - 导入不会在范围内带来隐含
问题描述
我implicit
在范围内面临一个关于 unreachable 的错误:
Error:(38, 68) could not find implicit value for parameter strategy: XXX.NeoStrategy[T]
(summoner: Summoner, v: String) => summoner.summonEvaluation[T](v)
我实现了蒂姆对该问题的回答:https ://stackoverflow.com/a/56668734/3896166
我试图implicit object
在范围内导入策略TypeTable
:
import XXX.NeoStrategies._
但没有成功。
以下是我要使用的基本逻辑的每个文件:
object TypeLib {
sealed trait Type_top
trait Type_A extends Type_top
trait Type_B extends Type_top
}
trait NeoStrategy[T <: Type_top] {
def evaluate(v: String, helper: Helper): Int
}
object NeoStrategies {
implicit object NeoStrategy_A extends NeoStrategy[Type_A] {
def evaluate(v: String, helper: Helper): Int = 1
}
implicit object NeoStrategy_B extends NeoStrategy[Type_B] {
def evaluate(v: String, helper: Helper): Int = 2
}
}
case class Helper(name: String) {
def summonEvaluation[T <: Type_top](v: String)(implicit strategy: NeoStrategy[T]): Int = {
strategy.evaluate(v, this)
}
}
trait TypeOMap {
protected def computeStuff[T <: Type_top]: (Helper, String) => Int
protected val computeMap: Map[String, (Helper, String) => Int]
}
import XXX.NeoStrategies._
trait TypeTable extends TypeOMap {
override protected def computeStuff[T <: Type_top]: (Helper, String) => Int = {
(helper: Helper, v: String) => helper.summonEvaluation[T](v)
}
override protected val computeMap = Map(
"a" -> computeStuff[Type_A],
"b" -> computeStuff[Type_B]
)
}
class Summoner extends TypeTable {
def callsMapAndEvaluates(typeIdentifier: String, helper: Helper, param: String): Double = {
computeMap(typeIdentifier)(helper, param)
}
}
object StackO {
def main(args: Array[String]): Unit = {
val mySummoner = new Summoner
// mySummoner allows the selecting of a given type with
// its "typeIdentifier" input in combination with the "TypeTable" it extends
val r = mySummoner.callsMapAndEvaluates("a", Helper("make it right"), "I, parameter")
}
}
这不是我第一次使用implicit
s 但不是像computeMap
上面这样的东西。不过,我理解它的逻辑,但未能使其正确。
我怎样才能summoner.summonEvaluation[T](v)
找到所需的implicit
?
解决方案
只需添加上下文绑定
override protected def computeStuff[T <: Type_top : NeoStrategy] ...
看来您想使用单例类型。在 Scala 2.12 + 无形中
import shapeless.Witness
object TypeLib {
sealed trait Type_top
trait Type_A extends Type_top
trait Type_B extends Type_top
}
import TypeLib._
trait NeoStrategy[S <: String] {
type T <: Type_top
def evaluate(v: S, summoner: Summoner): Int
}
object NeoStrategy {
type Aux[S <: String, T0 <: Type_top] = NeoStrategy[S] { type T = T0 }
def mkStrategy[S <: String, T0 <: Type_top](f: (S, Summoner) => Int): Aux[S, T0] = new NeoStrategy[S] {
override type T = T0
override def evaluate(v: S, summoner: Summoner): Int = f(v, summoner)
}
implicit val NeoStrategy_A: NeoStrategy.Aux[Witness.`"a"`.T, Type_A] = mkStrategy((_, _) => 1)
implicit val NeoStrategy_B: NeoStrategy.Aux[Witness.`"b"`.T, Type_B] = mkStrategy((_, _) => 2)
}
case class Summoner(name: String) {
def summonEvaluation[S <: String](s: Witness.Aux[S])(implicit
strategy: NeoStrategy[S]): Int = {
strategy.evaluate(s.value, this)
}
}
def main(args: Array[String]): Unit = {
val mySummoner = Summoner("stack question")
val r = mySummoner.summonEvaluation("a")
val r1 = mySummoner.summonEvaluation("b")
println(r) // 1
println(r1) // 2
}
在 Scala 2.13 中
object TypeLib {
sealed trait Type_top
trait Type_A extends Type_top
trait Type_B extends Type_top
}
import TypeLib._
trait NeoStrategy[S <: String with Singleton] {
type T <: Type_top
def evaluate(v: S, summoner: Summoner): Int
}
object NeoStrategy {
type Aux[S <: String with Singleton, T0 <: Type_top] = NeoStrategy[S] { type T = T0 }
def mkStrategy[S <: String with Singleton, T0 <: Type_top](f: (S, Summoner) => Int): Aux[S, T0] = new NeoStrategy[S] {
override type T = T0
override def evaluate(v: S, summoner: Summoner): Int = f(v, summoner)
}
implicit val NeoStrategy_A: NeoStrategy.Aux["a", Type_A] = mkStrategy((_, _) => 1)
implicit val NeoStrategy_B: NeoStrategy.Aux["b", Type_B] = mkStrategy((_, _) => 2)
}
case class Summoner(name: String) {
def summonEvaluation[S <: String with Singleton](s: S)(implicit
value: ValueOf[S],
strategy: NeoStrategy[S]): Int = {
strategy.evaluate(s, this)
}
}
def main(args: Array[String]): Unit = {
val mySummoner = Summoner("stack question")
val r = mySummoner.summonEvaluation("a")
val r1 = mySummoner.summonEvaluation("b")
println(r) // 1
println(r1) // 2
}
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