是否有一个类型类来检查是否存在至少一个隐式类型？

Question

I have a trait Foo[T, U] and a type-level algorithm that given an L <: HList and a target type U , tells me whether there exists T in L such that there is an implicit Foo[T, U] in scope. This is implemented using the following type class:

trait Search[L <: HList, U]

object Search {
  def apply[L <: HList, U](implicit s: Search[L, U]): U = null

  ...
}

and we have the following:

object Test {
  type L = Int :: String :: HNil

  implicit val foo: Foo[String, Boolean] = null

  Search[L, Boolean] //compiles

  Search[L, Double] //does not compile
}

What I would like is for the search to not take place at all if there is no Foo[T, U] for any T at all in scope, as then we already know that the algorithm will not complete. In other words, I want a type-class trait Exists[F[_]] for which instances exist if and only if there is at least one implicit F in scope, so the function Search.apply instead has signature:

def apply[L <: HList, U](implicit ev: Exists[Foo[_, U]], s: Search[L, U]): U = null

In this case the compiler will only try to resolve s if there is any implicit Foo in scope.

Is such a type-class possible to define? Does one already exist?

Answer 1

Try

import scala.language.experimental.macros
import scala.reflect.macros.{blackbox, contexts}

trait Exists[A]

object Exists {
  implicit def materialize[A]: Exists[A] = macro impl[A]

  def impl[A: c.WeakTypeTag](c: blackbox.Context): c.Tree = {
    import c.universe._
    val context = c.asInstanceOf[contexts.Context]
    val global: context.universe.type = context.universe
    val analyzer: global.analyzer.type = global.analyzer
    val callsiteContext = context.callsiteTyper.context

    val tpA = weakTypeOf[A]

    val searchResult = analyzer.inferImplicit(
      tree = EmptyTree.asInstanceOf[global.Tree],
      pt = tpA.asInstanceOf[global.Type],
      reportAmbiguous = false,
      isView = false,
      context = callsiteContext,
      saveAmbiguousDivergent = true,
      pos = c.enclosingPosition.asInstanceOf[global.Position]
    )

    val isAmbiguous = callsiteContext.reporter.firstError match {
      case Some(analyzer.AmbiguousImplicitTypeError(_,_)) => true
      case _ => false
    }

    if (searchResult.isSuccess || searchResult.isAmbiguousFailure || isAmbiguous) 
      q"new Exists[$tpA] {}"
    else c.abort(c.enclosingPosition, s"no implicit $tpA")    
  }
}

Test

// no implicit Int
// implicitly[Exists[Int]] // doesn't compile

implicit val i: Int = 1 
implicitly[Exists[Int]] // compiles

implicit val i: Int = 1 
implicit val i1: Int = 2 
implicitly[Exists[Int]] // compiles

I guess original Search was

trait Foo[U, V]

trait Search[L <: HList, V]

trait LowPrioritySearch {
  implicit def tail[H, T <: HList, V](implicit search: Search[T, V]): Search[H :: T, V] = null
}

object Search extends LowPrioritySearch {
  def apply[L <: HList, U](implicit s: Search[L, U]): U = null.asInstanceOf[U]

  implicit def head[U, T <: HList, V](implicit foo: Foo[U, V]): Search[U :: T, V] = null
}

Now with Exists

def apply[L <: HList, U](implicit ev: Exists[Foo[_, U]], s: Search[L, U]): U = null.asInstanceOf[U]

works as well

Search[L, Boolean] //compiles
// Search[L, Double] //does not compile

Tested in 2.13.0

libraryDependencies ++= Seq(
  scalaOrganization.value % "scala-reflect" % scalaVersion.value,
  scalaOrganization.value % "scala-compiler" % scalaVersion.value
)

Answer 2

In Scala 3, scala.util.Not (soon to be NotGiven ?), which exists if no implicits of the given type are found, can be used for this:

implicit val b: Byte = 1
  
summon[Not[Not[Byte]]] //compiles

implicit val i: Int = 0
implicit val i2: Int = 2

summon[Not[Not[Int]]] //compiles

summon[Not[Not[String]]] //doesn't compile - Not[String] not found

See it in Scastie .

Your Exists typeclass can now look like this (the syntax for givens may change soon, but you get the idea):

@annotation.implicitNotFound("No implicit of type ${T} was found")
trait Exists[T]
object Exists {
  given [T](using Not[Not[T]]) as Exists[T]
}

See it in Scastie .