型に数値を埋めこんでみよう ref: http://qiita.com/piyo7/items/5df3ad7e53216df5f65f

build.properties

sbt.version = 0.13.6

build.sbt

name := "coin"

version := "1.0"

scalaVersion := "2.11.2"

libraryDependencies += "com.github.okomok" % "sing-core_2.11" % "0.2.0"

file0.scala

case class Coin(value: Int, num: Int) {
  def +(a: Coin) = {
    assert(value == a.value, "違う種類の硬貨を足しています")
    Coin(value, num + a.num)
  }
  def -(a: Coin) = {
    assert(value == a.value, "違う種類の硬貨を引いています")
    Coin(value, num - a.num)
  }
  def sum = value * num
}

file1.scala

import com.github.okomok.sing.Dense

case class Coin[Value <: Dense] (value: Value, num: Int) {
  def +(a: Coin[Value]) = Coin(value, num + a.num)
  def -(a: Coin[Value]) = Coin(value, num - a.num)
  def sum = value.unsing * num // unsingでsing.Denseをscala.Intに変換。
}

file2.scala

import com.github.okomok.sing.Dense

val a: Coin[Dense._5] = Coin(Dense._5, 2) + Coin(Dense._5, 4)
// val b = Coin(Dense._5, 3) - Coin(Dense._10, 6) // コンパイル不可

file3.scala

import com.github.okomok.sing.Dense

class Coin[Value <: Dense] private(val value: Value, val num: Int) {
  def +(a: Coin[Value]) = new Coin(value, num + a.num)
  def -(a: Coin[Value]) = new Coin(value, num - a.num)
  def sum = value.unsing * num
}

object Coin {
  type _1 = Coin[Dense._1]
  def  _1(num: Int) = new _1(Dense._1, num)

  type _5 = Coin[Dense._5]
  def  _5(num: Int) = new _5(Dense._5, num)

  type _10 = Coin[Dense._10]
  def  _10(num: Int) = new _10(Dense._10, num)

  type _50 = Coin[Dense._50]
  def  _50(num: Int) = new _50(Dense._50, num)

  // singはDense._XXを50以下までしか定義していないので、
  // 自然数100を表す値と型を計算しておく。
  private type Dense_100 = Dense._50#times[Dense._2]
  private val  Dense_100 = Dense._50.times(Dense._2)

  type _100 = Coin[Dense_100]
  def  _100(num: Int) = new _100(Dense_100, num)

  // 同様に、自然数500を表す値と型を計算しておく。
  private type Dense_500 = Dense._50#times[Dense._10]
  private val  Dense_500 = Dense._50.times(Dense._10)

  type _500 = Coin[Dense_500]
  def  _500(num: Int) = new _500(Dense_500, num)
}

file4.scala

val a: Coin._1   = Coin._1(  1) // 1  円 x 1
val b: Coin._5   = Coin._5(  2) // 5  円 x 2
val c: Coin._10  = Coin._10( 3) // 10 円 x 3
val d: Coin._50  = Coin._50( 4) // 50 円 x 4
val e: Coin._100 = Coin._100(5) // 100円 x 5
val f: Coin._500 = Coin._500(6) // 500円 x 6

val aa: Coin._1 = a + a
// val ab = a + b // コンパイル不可

val wallet: Seq[Coin[_]] = Seq(a, b, c, d, e ,f)

println(wallet.map(_.sum).sum) // 3741円