Scala class extends {}

Question

By chance I came across weird compiling Scala syntax:

class Some extends {
  def hi = println(\"hi\")
}
         


        

Answer 1

This is actually a strange quirk in Scala's syntax. An extraneous extends is allowed before beginning the body of the class. Here are the relevant parts from the Scala Syntax Summary:

ClassDef          ::=  id [TypeParamClause] {ConstrAnnotation} [AccessModifier] 
                       ClassParamClauses ClassTemplateOpt 
ClassTemplateOpt  ::=  ‘extends’ ClassTemplate | [[‘extends’] TemplateBody]
ClassTemplate     ::=  [EarlyDefs] ClassParents [TemplateBody]

ClassTemplateOpt is everything after the class's parameters, in this case everything from extends onwards. The usual use of extends is the first alternation of ClassTemplateOpt, with extends being followed either by a parent or an early initializer. However, an early initializer cannot contain a def, and there is no way to interpret the contents of the braces as a parent. It cannot be a structural type because hi has a concrete definition.

The second alternation allows the class parameters to be immediately followed by the class body, without using extends. However, an optional extends is allowed. The extends in OP's code is an example of this, and is exactly equivalent to the same code without the optional extends:

class Some {
  def hi = println("hi")
}