Selecting type class instances by singleton types of values.

gistfile1.scala

scala> import shapeless._
import shapeless._

scala> import SingletonTypes._
import SingletonTypes._

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Show[T] {
  def show: String
}

object Show {
  implicit def showTrue[T](implicit t: T = refine(true)) = new Show[T] { def show = "true" }
  implicit def showFalse[T](implicit t: T = refine(false)) = new Show[T] { def show = "false" }

  implicit def showOne[T](implicit t: T = refine(1)) = new Show[T] { def show = "One" }
  implicit def showTwo[T](implicit t: T = refine(2)) = new Show[T] { def show = "Two" }
  implicit def showThree[T](implicit t: T = refine(3)) = new Show[T] { def show = "Three" }
}

// Exiting paste mode, now interpreting.

defined trait Show
defined module Show

scala> def show(t: Witness)(implicit s: Show[t.T]) = s.show
show: (t: shapeless.Witness)(implicit s: Show[t.T])String

scala> show(true)
res0: String = true

scala> show(false)
res1: String = false

scala> show(1)
res2: String = One

scala> show(2)
res3: String = Two

scala> show(3)
res4: String = Three

scala> show(4)
<console>:16: error: could not find implicit value for parameter s: Show[Int(4)]
              show(4)
                  ^

scala>