gistfile1.scala

/** Constructing "singleton types" from compile-time literals
  *
  * val x = Example.foo(42)
  * val y: x.T = 42 // compiles
  * val z: x.T = 43 // doesn't
  *
  */
package s

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

class Nat[In] { type T = In }

object NatMe {
  def foo(s: Int): Any = macro foo_impl
  def foo_impl(c: Context)(s: c.Expr[Int]) = {
    import c.universe._
    s.tree match {
      case Literal(const: Constant) => c.Expr(New(appliedType(typeOf[Nat[_]].typeConstructor, ConstantType(const) :: Nil)))
      case _                        => c.abort(c.enclosingPosition, "Not a literal")
    }
  }
}