In scala, is there any way to check if an instance is a singleton object or not?

Question

If I have an instance of an object, is there a way to check if I have a singleton object rather than an instance of a class? Is there any method can do this? May be some reflection API? I know that one difference is that the class name of a singleton object ends with a $, but this is not a strict way.

Answer 1

Yep, using the little-documented scala.Singleton type:

def isSingleton[A](a: A)(implicit ev: A <:< Singleton = null) = 
  Option(ev).isDefined

And then:

scala> val X = new Foo(10)
X: Foo = Foo@3d5c818f

scala> object Y extends Foo(11)
defined object Y

scala> isSingleton(X)
res0: Boolean = false

scala> isSingleton(Y)
res1: Boolean = true

My isSingleton method is just a demonstration that provides a runtime boolean value that tells you whether or not an expression is statically typed as a singleton type, but you can also use Singleton as evidence at compile time that a type is a singleton type.