Building up puppet variable based on conditions

So I'm getting adjusted to the immutable variables in puppet, now looking for some advice, or clarification to whether I'm already on the right track.

I have a directory I'd like to sync, but potentially a number of subdirectories thereof. So I'd like to build up an array of one or more paths to pass to a recursive file resource. Ideally, I'd have something like this

$paths = ['fist/path'] # assume this is provided via class parameter
# this should work, since it's only overriding what was provided from higher scope
if($condition) {
  $paths += 'second/path'
}
# this won't fly, since $paths has already been set once locally
if($another_condition) {
  $paths += 'third/path'
}

but I can't do that, right, since the variables are immutable. What I've come up with as a 'best' approach so far is something like this

$paths = ['fist/path']
if($condition) {
  $condition_1_path = ['second/path']
} else {
  $condition_1_path = []
}
if($another_condition) {
  $condition_2_path = ['third/path']
} else {
  $condition_2_path = []
}

$paths = concat($paths, $condition_1_path, $condition_2_path)

I'm unsure if concat will omit an entry from the result if it's provided an empty array for one of its arguments, but waiting to test that once I figure out how to get stdlib loaded.

Either way, looking at this code, to me, it's plain hideous. Is there a cleaner way to do something like this?