Terraform split/join/list

Question

I recently found the following line of terraform in our code (some values sanitized):

subnet_ids = [ "${split(",", var.xxx_lb ? join(",", data.yyy_ids.private.ids) : join(",", concat(data.yyy_ids.public.ids, list(""))))}" ]

I'm trying to understand why code would be written this way. More specifically, what is the final join doing? Pulling it out for clarity:

join(",", concat(data.yyy_ids.public.ids, list("")))

It seems that someone (no longer at the company) was trying to ensure that a non-empty list is returned. We definitely don't want the empty ("") item in the list.

So, the questions here are:

1. What logically is going on in this statement?
2. Is there a better way?
3. If there is not a better way, how can we remove the empty entry from the resulting list?

Answer 1

What logically is going on in this statement?

The original author attempts to create a list with the subnet ids

a) The first split statement will take a string in this case the subnet ids and return them as a list, by splitting them based on the delimiter ,

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var.xxx_lb ? clause_if_true : clause_if_false

b) Next terraform will evaluate this variable as a boolean and according to the result you will get the public or the private subnet ids, by employing the ternary operator syntax

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join(",", data.yyy_ids.private.ids)

c) In case the boolean value is true, terraform will examine this part

This will return a string by joining the items of the list. And add the delimeter ,. I assume the reason that he attempts to join them as a string is to be accordance with the section a)

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join(",", concat(data.yyy_ids.public.ids, list("")))

d) If the boolean value in b) evaluates to false terraform will examine this part. The concat function takes as input lists and returns them as a single list.

And then performs the same logic as in c)

The list function is deprecated, tolist should be used instead.

Is there a better way?

I would employ a straight forward way. Check the boolean value, if it is true get the list with private ids. If false the public ones.

subnet_ids =  var.xxx_lb ? data.yyy_ids.private.ids : data.yyy_ids.public.ids