Question

Trying to grok this comment, I wrote the following code:

def breakfast : AnyRef = {

    class Chicken  (e: =>Egg) { 
      lazy val offspring = e 
    }

    class Egg (c: =>Chicken) {
      lazy val mother = c
    }
   lazy val (egg: Egg, chicken: Chicken) = (new Egg(chicken), 
new Chicken(egg))
  egg  
}

And it works and it does exactly what you’d hope it’d do. What I don’t get is, why is the : AnyRef necessary? If it’s not included, the compiler (the 2.8 compiler at least) dies a horrible death:

error: type mismatch; found : Egg(in lazy value scala_repl_value)
where type Egg(in lazy value scala_repl_value) <: java.lang.Object
with ScalaObject{lazy def mother: Chicken} required: (some
other)Egg(in lazy value scala_repl_value) forSome { type (some
other)Egg(in lazy value scala_repl_value) <: java.lang.Object with
ScalaObject{lazy def mother: Chicken}; type Chicken <:
java.lang.Object with ScalaObject{lazy def offspring: (some
other)Egg(in lazy value scala_repl_value)} } object
RequestResult$line16$object {

Can somebody explain what’s going on here?

Answer 1

You define classes Chicken and Egg in scope of breakfast function, so they are not seen outside i.e. nobody except breakfast don’t know these classes. In Scala 2.9 this snippet actually works, and breakfast function’s return value is defined as

def breakfast: Egg forSome { type Egg <: java.lang.object with scalaobject{lazy val mother: chicken}; type chicken <

When classes defined outside of function everything works as expected

class Chicken(e: => Egg) {
  lazy val offspring = e
}

class Egg(c: => Chicken) {
  lazy val mother = c
}

def breakfast: Egg = {

  lazy val (egg: Egg, chicken: Chicken) =
    (new Egg(chicken), new Chicken(egg))

  egg
}