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flatten Function

flatten takes a list and replaces any elements that are lists with a flattened sequence of the list contents.


Code Block
> flatten([["a", "b"], [], ["c"]])
["a", "b", "c"]

If any of the nested lists also contain directly-nested lists, these too are flattened recursively:

Code Block
> flatten([[["a", "b"], []], ["c"]])
["a", "b", "c"]

Indirectly-nested lists, such as those in maps, are not flattened.

Flattening nested structures for for_each

The resource for_each and dynamic block language features both require a collection value that has one element for each repetition.

Sometimes your input data structure isn't naturally in a suitable shape for use in a for_each argument, and flatten can be a useful helper function when reducing a nested data structure into a flat one.

For example, consider a module that declares a variable like the following:

Code Block
variable "networks" {
type = map(object({
cidr_block = string
subnets = map(object({ cidr_block = string }))

The above is a reasonable way to model objects that naturally form a tree, such as top-level networks and their subnets. The repetition for the top-level networks can use this variable directly, because it's already in a form where the resulting instances match one-to-one with map elements:

Code Block
resource "aws_vpc" "example" {
for_each = var.networks

cidr_block = each.value.cidr_block

However, in order to declare all of the subnets with a single resource block, we must first flatten the structure to produce a collection where each top-level element represents a single subnet:

Code Block
locals {
# flatten ensures that this local value is a flat list of objects, rather
# than a list of lists of objects.
network_subnets = flatten([
for network_key, network in var.networks : [
for subnet_key, subnet in network.subnets : {
network_key = network_key
subnet_key = subnet_key
network_id = aws_vpc.example[network_key].id
cidr_block = subnet.cidr_block

resource "aws_subnet" "example" {
# local.network_subnets is a list, so we must now project it into a map
# where each key is unique. We'll combine the network and subnet keys to
# produce a single unique key per instance.
for_each = {
for subnet in local.network_subnets : "${subnet.network_key}.${subnet.subnet_key}" => subnet

vpc_id = each.value.network_id
availability_zone = each.value.subnet_key
cidr_block = each.value.cidr_block

The above results in one subnet instance per subnet object, while retaining the associations between the subnets and their containing networks.

  • setproduct finds all of the combinations of multiple lists or sets of values, which can also be useful when preparing collections for use with for_each constructs.