How to define a list in Terraform that may be empty depending on a variable?

Question

I need to define a resource in Terraform (v0.10.8) that has a list property that may or may not be empty depending on a variable, see volume_ids in the following definition:

resource "digitalocean_droplet" "worker_node" {
  count = "${var.droplet_count}"
  [...]
  volume_ids = [
    "${var.volume_size != 0 ? element(digitalocean_volume.worker.*.id, count.index) : ""}"
  ]
}

resource "digitalocean_volume" "worker" {
  count = "${var.volume_size != 0 ? var.droplet_count : 0}"
  [...]
}

}

The solution I've come up with fails however in the case where the list should be empty (i.e., var.volume_size is 0):

volume_ids = [
  "${var.volume_size != 0 ? element(digitalocean_volume.worker.*.id, count.index) : ""}"
]

The following Terraform error message is produced:

* module.workers.digitalocean_droplet.worker_node[1]: element: element() may not be used with an empty list in:

${var.volume_size != 0 ? element(digitalocean_volume.worker.*.id, count.index) : ""}

How should I correctly write my definition of volume_ids?

RaGe · Accepted Answer · 2017-11-21 14:40:06Z

13

Unfortunately this is one of many language shortcomings in terraform. The hacky workaround is to tack an empty list onto your empty list.

${var.volume_size != 0 ? element(concat(digitalocean_volume.worker.*.id , list("")), count.index) : ""}

answered Nov 21, 2017 at 14:40

RaGe

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2 Comments

aknuds1 Over a year ago

I found out about this in the meantime too, thanks. I think you can remove the boolean condition though? element will wrap around the index if it surpasses the list's length, so just looking up list("") should work in case there are no volumes .

Asfand Qazi Over a year ago

The truth, of course, is that Terraform is not a language, it is a config file format. What we need are tools that wrap Terraform, like stack_master/stacker/etc. for CloudFormation, which let you code in proper programming languages.

Tom Harrison · Accepted Answer · 2022-06-14 17:03:32Z

3

Since this answer was written, several new constructs make this problem just a little less gross. As of terraform 1.x syntax can be changed from:

element(concat(digitalocean_volume.worker.*.id , list(""))

coalescelist(digitalocean_volume.worker[*].id, [""])

answered Jun 14, 2022 at 17:03

Tom Harrison

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