Restrict Scala type to be one of a set of types

Question

Is there a way to define a generic type parameter that can be one of a small set of types? I want to define a type T that can only be one of {Int, Long, Float, Double}.

Answer 1

Daniel Landgon's comment above points in the right direction.
In case someone is in a hurry to click the link though:

@annotation.implicitNotFound("It can not be proven that ${T} is of type Int, Long, Float or Double")
sealed trait Foo[T]

object Foo {
  implicit val intFoo: Foo[Int] = new Foo[Int]{}  
  implicit val LongFoo: Foo[Long] = new Foo[Long]{}
  implicit val FloatFoo: Foo[Float] = new Foo[Float]{}
  implicit val DoubleFoo: Foo[Double] = new Foo[Double]{}
}

With:

def bar[T](t: T)(implicit ev: Foo[T]): Unit = println(t)

We get:

bar(5)        // res: 5
bar(5.5)      // res: 5.5
bar(1.2345F)  // res: 1.2345

bar("baz")    // Does not compile. Error: "It can not be proven that String is of type Int, Long, Float or Double"
bar(true)     // Does not compile. Error: "It can not be proven that Boolean is of type Int, Long, Float or Double"

This could also be achieved with Miles Sabin's union type as provided in his shapeless library.