some of NewType

newtype.scala

object TaggedTypes {
	sealed trait TaggedType[T] {
		def underlying: T
		override def toString = underlying.toString
	}
	//implicitly extract the primitive value from a TaggedType[T]
	implicit def underlying[T](w: TaggedType[T]): T = {
		w.underlying
	}
	
	//an optional TaggedType. used on construction of a NewType
	type MbTaggedType[T] = Option[TaggedType[T]]
	
	//extension methods on MbTaggedType
	sealed trait MbTaggedTypeW[T] {
		protected def opt: MbTaggedType[T]
		def underlying: Option[T] = opt.map(_.underlying)
	}
	implicit def optWrapperToW[T](o: MbTaggedType[T]) = new MbTaggedTypeW[T] {
		override lazy val opt = o
	}
	
	//convert a MbTaggedType[T] to an Option[T]
	implicit def mbTaggedTypeOpt[T](mbTaggedType: MbTaggedType[T]): Option[T] = mbTaggedType.map(_.underlying)
		
	private def wrapper[T](t: T): TaggedType[T] = new TaggedType[T] {
		override lazy val underlying = t
	}
	
	//the constructor for new type that's based on a primitive, plus some constraints on that primitive
	type TaggedTypeConstructor[T] = T => MbTaggedType[T]
	//create a function that creates a MbTaggedType[T] based on the given isValid method
	def NewType[T](isValid: T => Boolean): T => MbTaggedType[T] = { value: T =>
		if(isValid(value)) {
			Some(wrapper(value))
		} else {
			None
		}
	}
	//create a function that creates a TaggedType[T] or throws based on the given isValid method.
	//I don't like throwing. This method is here for practicality
	def NewTypeOrThrow[T](isValid: T => Boolean): T => TaggedType[T] = { value: T =>
		if(isValid(value)) { wrapper(value) } else { throw new Exception("invalid value %s".format(value.toString)) }
	}
}

//a type that holds a positive integer only
import TaggedTypes._
type AppId = TaggedType[Int]
val AppId: TaggedTypeConstructor[Int] = NewType { i: Int => i > 0 }

val mbAppId1: Option[AppId] = AppId(0)
val mbAppId2: Option[AppId] = AppId(2)
val mbAppId3: Option[AppId] = AppId(-1)

println("appId1 = %s".format(mbAppId1.underlying))
println("appId2 = %s".format(mbAppId2.toString))
println("appId3 = %s".format(mbAppId3: Option[Int]))

output is

appId1 = None
appId2 = Some(2)

output is now

appId1 = None
appId2 = Some(2)
appId3 = None