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Wisely or not, I'm writing a method that I'd like to accept only Scala singletons, i.e. objects implemented via "object" rather than constructed instances of a class or trait. It should accept Scala singletons of any type, so "MySingleton.type" won't do. I came upon the very strange construct "scala.Singleton", which is not documented in the api docs, but seems to do the trick: scala> def check( obj : Singleton ) = obj check: (obj: Singleton)Singleton scala> check( Predef ) res0: Singleton = scala.Predef$@4d3e9963 scala> check ( new java.lang.Object() ) <console>:9: error: type mismatch; found : java.lang.Object required: Singleton check ( new java.lang.Object() ) scala> check( Map ) res3: Singleton = scala.collection.immutable.Map$@6808aa2d scala> check( Map.empty[Any,Any] ) <console>:9: error: type mismatch; found : scala.collection.immutable.Map[Any,Any] required: Singleton check( Map.empty[Any,Any] ) However, rather inexplicably (to me), String literals are accepted as Singletons while explicitly constructed Strings are not: scala> check( "foo" ) res7: Singleton = foo scala> check( new String("foo") ) <console>:9: error: type mismatch; found : java.lang.String required: Singleton check( new String("foo") ) Why do String literals conform to Singleton? Am I misunderstanding what the Singleton type is supposed to specify?

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