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Terraform List

All code below also in the main.tf in the root module, unless stated otherwise.

List we use​

The List we are working with

variable "LIST_1" {
  type = list(string)
  default = [
    "ValueA",
    "ValueB",
    "ValueC"
  ]
}

Get the second element value from the List​

output "OUT_LIST_1_INDEX_2" {
  value = var.LIST_1[2]
}

Output​

OUT_LIST_1_INDEX_2 = "ValueC"

Get all values from the List​

Here we use a loop in the value.

output "OUT_LIST_1_ALL_VALUES_IN_A_FOR" {
  value = [for i in var.LIST_1 : i]
}

Output​

OUT_LIST_1_ALL_VALUES_IN_A_FOR = [
  "ValueA",
  "ValueB",
  "ValueC",
]

We're getting a nice list here.

Get all values from the list, without 'for'​

Well, what happens if we don't use the 'for' loop

output "OUT_LIST_1_ALL_VALUES" {
  value = var.LIST_1
}

Output​

OUT_LIST_1_ALL_VALUES = tolist([
  "ValueA",
  "ValueB",
  "ValueC",
])

Per Element (in a Module)​

As we did for the Map, lets create a Module in which we process each element of the List.

Calling the Module​

In the root module main.tf file:

module "MODULE_LIST" {
  source       = "./module_list_element"
  count        = length(var.LIST_1)
  LIST_ELEMENT = var.LIST_1[count.index]
}

Since this is a list, we call the module as many times as we have elements in the list. And that is done with a 'count' which is given a value of the length of the list.

It's like in Javascript:

for (let i = 0; i < LIST_1.length; i++) {
}

The variable which we send to the module, called LIST_ELEMENT, passes the index of that counter.

(Intermezzo) : Why not for_each?​

Well, for_each does not work on Lists. If you try the following:

module "MODULE_LIST_2" {
  source       = "./module_list_element"
  for_each = var.LIST_1
  LIST_ELEMENT = each.key
}

You will get a nice error saying:

The given "for_each" argument value is unsuitable:
the "for_each" argument must be a map, or set of strings,
and you have provided a value of type list of string.

Back to our Module​

The Module definition looks like this: (located in ./module_list_element/main.tf)

variable "LIST_ELEMENT" {
  type = string
}

output "MODULE_LIST_ELEMENT_OUTPUT" {
  value = var.LIST_ELEMENT
}

It receives a LIST_ELEMENT and sends back that same LIST_ELEMENT in an output called MODULE_LIST_ELEMENT_OUTPUT.

Show the output for the module, all occurrences​

We can do various outputs with that Module

output "MODULE_LIST_ELEMENT_OUTPUT" {
  value = module.MODULE_LIST
}

Output​

MODULE_LIST_ELEMENT_OUTPUT = [
  {
    "MODULE_LIST_ELEMENT_OUTPUT" = "ValueA"
  },
  {
    "MODULE_LIST_ELEMENT_OUTPUT" = "ValueB"
  },
  {
    "MODULE_LIST_ELEMENT_OUTPUT" = "ValueC"
  },
]

Return the value of the second object in the list​

output "MODULE_LIST_ELEMENT_OUTPUT_SECOND_VALUE" {
  value = module.MODULE_LIST[2].MODULE_LIST_ELEMENT_OUTPUT
}

Output​

MODULE_LIST_ELEMENT_OUTPUT_SECOND_VALUE = "ValueC"

Return all values in a list​

This is called a 'splat' expression in Terraform

output "MODULE_LIST_ELEMENT_OUTPUT_ALL_IN_LIST" {
  value = module.MODULE_LIST[*].MODULE_LIST_ELEMENT_OUTPUT
}

The earlier versions of Terraform a deprecated form of the Splat expression was without the bracket.

module.MODULE_LIST.*.MODULE_LIST_ELEMENT_OUTPUT

It's advised not to use that anymore.

Output​

MODULE_LIST_ELEMENT_OUTPUT_ALL_IN_LIST = [
  "ValueA",
  "ValueB",
  "ValueC",
]