Puppet users - Feb 2014 - Condition on class existence on agent

Hi,
how can I create a condition based on the presence of a specific puppet 
class on the host?

I'm using Foreman, some hosts have myclass_a applied, while others have 
myclass_a AND myclass_b.
Resources in myclass_b are applied after resources in myclass_a.

Both classes change a parameter inside a configuration file, using augeas. 
But this causes two changes in the file on each puppet run (from state b to 
a, and then from state a to b again).

So I'm trying to do something like this, to prevent myclass_a from applying 
the modification if also myclass_b is present:

class myname::myclass_a {

  if defined("myname::myclass_b") {
    $confchanges = [
                     'set anotherpath_b anothervalue_b', # if class B is
present, I only apply my specific edits
                    ]
  }
  else {
    $confchanges = [
                     'set anotherpath_b anothervalue_b',
                     'set mypath myvalue_a', # if class B is absent, I
also
apply the conflicting edit
                    ]
  }

  augeas { 'myconffile_a':
    lens    => 'Puppet.lns',
    incl    => '/etc/myconf.conf',
    changes => $confchanges,
  }

}

class myname::myclass_b {

  augeas { 'myconffile_b':
    lens    => 'Puppet.lns',
    incl    => '/etc/myconf.conf',
    changes => 'set mypath myvalue_b',
  }

}

...but this does not work, it never gets to the "else" section.
Probably
because the "defined" function is evaluated on the puppet master,
where the
class always exists.

I cannot use a condition on the existence of the "myconffile_b"
resource,
because this is parse order dependent and would always return false when 
evaluated inside myclass_a.

Any suggestions?
I'm looking into alternatives like virtual resources or defined types, but 
I cannot see anything better suited at the moment.

Thanks.
Marco

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