Question

The Scala Language Specification says under §3.2.1:

A stable type is either a singleton
type or a type which is declared to be
a subtype of trait scala.Singleton.

I couldn’t find scala.Singleton neither in the sources, in ScalaDoc nor in the binary jar file.

Trying on the REPL results in:

scala> class Foo extends Singleton
<console>:9: error: illegal inheritance from final trait Singleton
       class Foo extends Singleton
                         ^
<console>:9: error: illegal inheritance; superclass Any
 is not a subclass of the superclass Object
 of the mixin trait ScalaObject
       class Foo extends Singleton
                 ^

Where does scala.Singelton live and what’s its purpose?

Answer 1

Finally found something on this:

The type Singleton is essentially an
encoding trick for existentials with
values. I.e.

T forSome { val x: T } 

is turned into

[x.type := X] T forSome { type X <:T with Singleton } 

Source: http://scala-programming-language.1934581.n4.nabble.com/scala-Singleton-td1940630.html