Implicit Evidence Summoning

ImplicitSummoner.scala

import scala.language.experimental.macros
import scala.language.higherKinds

/**
 * @author Tongfei Chen
 */
trait ImplicitSummoner1[Ev[_]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A](implicit ev: Ev[A]): Ev[A] = macro ImplicitSummoning.summon1[Ev, A]
}


trait ImplicitSummoner2[Ev[_, _]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A, B](implicit ev: Ev[A, B]): Ev[A, B] = macro ImplicitSummoning.summon2[Ev, A, B]
}

trait ImplicitSummoner3[Ev[_, _, _]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A, B, C](implicit ev: Ev[A, B, C]): Ev[A, B, C] = macro ImplicitSummoning.summon3[Ev, A, B, C]
}

trait ImplicitSummonerH1[Ev[_[_]]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A[_]](implicit ev: Ev[A]): Ev[A] = macro ImplicitSummoning.summonH1[Ev, A]
}

trait ImplicitSummonerH2[Ev[_[_, _]]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A[_, _]](implicit ev: Ev[A]): Ev[A] = macro ImplicitSummoning.summonH2[Ev, A]
}

trait ImplicitSummonerH1H1[Ev[_[_], _[_]]] {
  /**
   * Retrieves the implicit algebraic structure implicitly defined in this scope.
   * @param ev The implicit object to be retrieved
   */
  def apply[A[_], B[_]](implicit ev: Ev[A, B]): Ev[A, B] = macro ImplicitSummoning.summonH1H1[Ev, A, B]
}

ImplicitSummoning.scala

import scala.reflect.macros._
import scala.language.higherKinds

/**
 * Enables zero-overhead summoning of implicit typeclass instances (as
 * compared to the standard [[implicitly]], which has a method call overhead).
 * @author Tongfei Chen
 */
object ImplicitSummoning {
  def summon1[Ev[_], A](c: Context)(ev: c.Expr[Ev[A]]): c.Expr[Ev[A]] = ev
  def summon2[Ev[_, _], A, B](c: Context)(ev: c.Expr[Ev[A, B]]): c.Expr[Ev[A, B]] = ev
  def summon3[Ev[_, _, _], A, B, C](c: Context)(ev: c.Expr[Ev[A, B, C]]): c.Expr[Ev[A, B, C]] = ev
  def summonH1[Ev[_[_]], A[_]](c: Context)(ev: c.Expr[Ev[A]]): c.Expr[Ev[A]] = ev
  def summonH2[Ev[_[_, _]], A[_, _]](c: Context)(ev: c.Expr[Ev[A]]): c.Expr[Ev[A]] = ev
  def summonH1H1[Ev[_[_], _[_]], A[_], B[_]](c: Context)(ev: c.Expr[Ev[A, B]]): c.Expr[Ev[A, B]] = ev
}

usage.scala

object Eq extends ImplicitSummoner1[Eq]
object MetricSpace extends ImplicitSummoner2[MetricSpace]
object Functor extends ImplicitSummonerH1[Functor]
object Arrow extends ImplicitSummonerH2[Arrow]