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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.

Examples

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

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

> 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:

variable "networks" {  type = map(object({    cidr_block = string    subnets    = map(object({ cidr_block = string }))  }))  default = {    "private" = {      cidr_block = "10.1.0.0/16"      subnets = {        "db1" = {          cidr_block = "10.1.0.1/16"        }        "db2" = {          cidr_block = "10.1.0.2/16"        }      }    },    "public" = {      cidr_block = "10.1.1.0/16"      subnets = {        "webserver" = {          cidr_block = "10.1.1.1/16"        }        "email_server" = {          cidr_block = "10.1.1.2/16"        }      }    }    "dmz" = {      cidr_block = "10.1.2.0/16"      subnets = {        "firewall" = {          cidr_block = "10.1.2.1/16"        }      }    }  }}

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:

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:

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.

Related Functions

  • 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.