puppetlabs/stdlib and delete on undef elements

stdlib-delete-test.pp

$array1 = undef
$array2 = ['a','b','c']
$array3 = [$array1, $array2]
$flat = flatten([$array3])
$pruned = delete($flat,undef)
$plist = join($pruned,',')
notice("Pruned array contains ${plist}")

# Expected Result - order not guaranteed
# "Pruned array contains a,b,c"
# Actual Results - order not guaranteed
# "Pruned array contains a,b,c,undef"

# The actual code snippet that is failing for me is
  if $hostgroups and is_array($hostgroups) {
    $prune_undef = delete($hostgroups,undef)
    $prune_undef_str = delete($prune_undef,'undef')
    $hlist = join($prune_undef_str,',')
  } elsif $hostgroups {
    notice("Received hostgroups as a string")
    $hlist = $hostgroups
  }
  notice("Hostgroups to apply for ${host_name} are ${hlist}")
# I still get a list that includes the string "undef" in the array

I believe my solution is to add a known string, which I can prune later.

# $hostgroups is passed in as an array at the class
$not_present = 'not_present'
$hg_vmware_guest = $::virtual ? {
    /vmware/ => 'vmware_guest',
    default     => $not_present
}
$hg_os_linux = $::kernel ? {
    /Linux/ => 'os_linux',
    default => $not_present,
}
$flat = flatten([$hg_vmware_guest, $hg_os_linux, $hostgroups])
$uniq = unique($flat)
$hg = delete($uniq,$not_present)
$hlist = join($hg,',')

nagios_host { $::hostname: hostgroups => $hlist }

I think this calls for adding a function called 'append'