ifrepo.pp

  if manage_repo
    yumrepo {'repo':
      enabled  => 1,
      gpgcheck => 1,
    }
  end
  
  package { 'package':
    ensure  => latest,
    if Yumrepo['repo'] was defined:
      require => Yumrepo['repo'],
    end
  }

if manage_repo
   yumrepo{ ....}

   # its called 'setting defaults' in the docs
   Package { require => Yumrepo[...]}
end