Summon a conditional implicit

conditional.scala

/*
 * implicit conversion ("view"): implicit def f(x: A): B
 * non-view implicit:            implicit def f(implicit x: A): B
 *
 * we can summon implicit conversions with `implicitly[A => B]`
 * we can't do that with non-view implicits
 * this macro provides the trait `Conditional[A, B]` that provides precisely that
 */

package conditional

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context

trait Conditional[A, B] {
  def apply(a: A): B
}

object Conditional {

  def impl[A : c.WeakTypeTag, B : c.WeakTypeTag](c: Context) = {
    import c.universe._

    val a = weakTypeOf[A]
    val b = weakTypeOf[B]

    val tree = q"""
      new _root_.conditional.Conditional[$a, $b] {
        def apply(a: $a): $b = {
          implicit val _a: $a = a
          _root_.scala.Predef.implicitly[$b]
        }
      }
    """

    c.Expr[Conditional[A, B]](tree)
  }

  implicit def summon[A, B]: Conditional[A, B] = macro impl[A, B]

}

Wait, I think this can actually be a blackbox macro. No need for whitebox here. It can.

You should use c.freshName to generate identifiers for a and _a in case these might shadow (badly named!) implicits you want implicitly to find.