How to Use Flatten Function in Terraform: Syntax and Examples

In Terraform, the flatten function takes a list of lists and merges them into a single list by removing one level of nesting. Use it when you have nested lists and want to simplify them into one list for easier processing.

Syntax

The flatten function syntax is simple: it takes one argument, which must be a list of lists, and returns a single list with all the inner elements combined.

• flatten(list_of_lists): The input is a list where each element is itself a list.
• The output is a single list containing all elements from the inner lists in order.

terraform

flatten([["a", "b"], ["c", "d"], ["e"]])

Output

["a", "b", "c", "d", "e"]

Example

This example shows how to use flatten to combine nested lists of strings into one list. It demonstrates how Terraform processes the nested structure.

terraform

variable "nested_list" {
  default = [["apple", "banana"], ["cherry", "date"], ["fig"]]
}

output "flat_list" {
  value = flatten(var.nested_list)
}

Output

flat_list = ["apple", "banana", "cherry", "date", "fig"]

Common Pitfalls

Common mistakes when using flatten include:

• Passing a list that is not nested (e.g., a flat list) will cause an error because flatten expects a list of lists.
• Trying to flatten more than one level of nesting at once is not possible; flatten only removes one level.
• Confusing flatten with concat, which joins lists but does not flatten nested lists.

terraform

/* Wrong: flatten on a flat list causes error */
# flatten(["a", "b", "c"])

/* Right: flatten on a list of lists */
flatten([["a"], ["b", "c"]])

Quick Reference

flatten function summary:

• flatten(list_of_lists) → single list
• Removes one level of nesting
• Input must be a list of lists
• Use when you want to simplify nested lists

Key Takeaways

Use flatten to convert a list of lists into a single flat list by removing one nesting level.

The input to flatten must be a nested list; passing a flat list causes errors.

flatten only removes one level of nesting, not multiple levels.

Do not confuse flatten with concat; flatten merges nested lists, concat joins lists.

Use flatten to simplify complex list structures for easier Terraform processing.