Puppet

Current inheritance behaviour doesn’t cover enough functionality. It’s quite often that people want to override things other than resources in a subclass, but it doesn’t work. E.g.:

class base {
  $var = foo
  file { "/m": content => $var }
}
class sub inherits base {
  $var = other
}

This doesn’t work for two reasons: First, because the resource is created with the base class’s value before the sub class sets its variable. This will be fixed in #2596. Second, the base and subclass have separate scopes, so the resource defined in the base class knows absolutely nothing about the subclass’s variable.

I propose we add a new kind of class relationship, called ‘extension’. It’s very similar to inheritance (and likely will entirely replace it at some point), except that the entire class hierarchy shares a single scope. Thus, in the above code, the subclass’s $var setting would actually replace the base class’s setting.

This behaviour would require that all things that affect scope — variable setting, defaults, and maybe more — would need to retain source information, like resource parameters do, so we could meet the same inheritance rules we have now. For instance, the following should be an error:

class base {
  $var = foo
  file { "/m": content => $var }
}
class sub1 extends base {
  $var = other
}
class sub2 extends base {
  $var = yay
}
include sub1, sub2

But this would not:

class base {
  $var = foo
  file { "/m": content => $var }
}
class sub1 extends base {
  $var = other
}
class sub2 extends sub1 {
  $var = yay
}
include sub1, sub2

This works because sub2 is a child of sub1, rather than a peer, so it can override its parent.

It’s currently unresolved whether extension and inheritance can be used in the same class hierarchy. Instinctively, it seems like they could, but I think that would be pretty confusing.