terraform/website/docs/language/functions/flatten.mdx

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---
page_title: flatten - Functions - Configuration Language
description: The flatten function eliminates nested lists from a list.
---
# `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`](/language/meta-arguments/for_each)
and
[`dynamic` block](/language/expressions/dynamic-blocks)
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:
```hcl
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:
```hcl
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:
```hcl
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`](/language/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.