Singleton

final open trait Singleton

Singleton is used by the compiler as a supertype for singleton types. This includes literal types, as they are also singleton types.

scala> object A { val x = 42 }
defined object A

scala> implicitly[A.type <:< Singleton]
res12: A.type <:< Singleton = generalized constraint

scala> implicitly[A.x.type <:< Singleton]
res13: A.x.type <:< Singleton = generalized constraint

scala> implicitly[42 <:< Singleton]
res14: 42 <:< Singleton = generalized constraint

scala> implicitly[Int <:< Singleton]
^
error: Cannot prove that Int <:< Singleton.

Singleton has a special meaning when it appears as an upper bound on a formal type parameter. Normally, type inference in Scala widens singleton types to the underlying non-singleton type. When a type parameter has an explicit upper bound of Singleton, the compiler infers a singleton type.

scala> def check42[T](x: T)(implicit ev: T =:= 42): T = x
check42: [T](x: T)(implicit ev: T =:= 42)T

scala> val x1 = check42(42)
^
error: Cannot prove that Int =:= 42.

scala> def singleCheck42[T <: Singleton](x: T)(implicit ev: T =:= 42): T = x
singleCheck42: [T <: Singleton](x: T)(implicit ev: T =:= 42)T

scala> val x2 = singleCheck42(42)
x2: Int = 42

class Any