CODEWITHSUNDEEP
scala
Michael Lorton
Michael Lorton

Reputation: 44436

Explain this type-mismatch in lazy evaluation

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?

Upvotes: 4

Views: 756

Answers (1)

4e6
4e6

Reputation: 10776

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
}

Upvotes: 4

Related Questions