Issue #2957 has been reported by Luke Kanies.
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Feature #2957: Classes should support an 'extends' functionality
http://projects.reductivelabs.com/issues/2957
Author: Luke Kanies
Status: Accepted
Priority: Normal
Assigned to:
Category: language
Target version: Rowlf
Affected version: 0.25.1
Keywords:
Branch:
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.:
<pre>
class base {
$var = foo
file { "/m": content => $var }
}
class sub inherits base {
$var = other
}
</pre>
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:
<pre>
class base {
$var = foo
file { "/m": content => $var }
}
class sub1 extends base {
$var = other
}
class sub2 extends base {
$var = yay
}
include sub1, sub2
</pre>
But this would not:
<pre>
class base {
$var = foo
file { "/m": content => $var }
}
class sub1 extends base {
$var = other
}
class sub2 extends sub1 {
$var = yay
}
include sub1, sub2
</pre>
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.
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