Import does not bring implicits in scope - scala

Question

I am facing an error about unreachable implicits in scope:
Error:(38, 68) could not find implicit value for parameter strategy: XXX.NeoStrategy[T]
(summoner: Summoner, v: String) => summoner.summonEvaluation[T](v)
I implement the answer of Tim to that question : https://stackoverflow.com/a/56668734/3896166
I tried to import the implicit object Strategies within TypeTable scope with :
import XXX.NeoStrategies._
but to no success.
The followings are each file of the base logic I want to use:
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")
}
}
This is not the first time I use implicits but not with something like the computeMap above. Still, I understand the logic of it, but fail at making it right.
How can I have summoner.summonEvaluation[T](v) find the required implicit?

Answer 1

Just add context bound
override protected def computeStuff[T <: Type_top : NeoStrategy] ...
It seems you want to work with singleton types. In Scala 2.12 + Shapeless
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
}
In Scala 2.13
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
}

Answer 2

The underlying problem is this:
override protected def computeStuff[T <: Type_top]: (Helper, String) => Int = {
(helper: Helper, v: String) => helper.summonEvaluation[T](v) // implicit for NeoStrategy[T]...?
}
Since summonEvaluation[T] requires an implicit argument of type NeoStrategy[T], this means you must have one in scope for any T that's a subclass of Type_top. However, NeoStrategies only provides two instances: one for Type_A and Type_B. This is not enough for the compiler. Understandably so - for instance, you haven't provided any NeoStrategy for
Type_top itself
subclasses of Type_A and Type_B (perfectly legal to create)
There are two basic ways you can handle this:
Delaying the implicit resolution
As per the other answer, instead of trying to resolve the implicit inside computeStuff, add a context bound there too. If the point where you have to supply the implicit is only reached when you know what T is, you won't have to provide instances for any possible subtype.
Providing implicits for all possible subtypes
If absolutely you want to keep the implicit resolution inside computeStuff, you're going to have to offer a method
implicit def getNeoStrategy[T <: Type_top] : NeoStrategy[T] = ???
Unfortunately, doing this is probably going to involve a bunch of reflection and potentially runtime errors for edge cases, so I'd recommend the context bound on computeStuff.