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Local Values

A local value assigns a name to an expression, so you can use the name multiple times within a module instead of repeating the expression.

If you're familiar with traditional programming languages, it can be useful to compare modules to function definitions:

  • Input variables are like function arguments.
  • Output values are like function return values.
  • Local values are like a function's temporary local variables.

Declaring a Local Value

A set of related local values can be declared together in a single locals block:

Code Block
locals {
service_name = "forum"
owner = "Community Team"

The expressions in local values are not limited to literal constants; they can also reference other values in the module in order to transform or combine them, including variables, resource attributes, or other local values:

Code Block
locals {
# Ids for multiple sets of EC2 instances, merged together
instance_ids = concat(*.id,*.id)

locals {
# Common tags to be assigned to all resources
common_tags = {
Service = local.service_name
Owner = local.owner

Using Local Values

Once a local value is declared, you can reference it in expressions as local.<NAME>.

Code Block
resource "aws_instance" "example" {
# ...

tags = local.common_tags

A local value can only be accessed in expressions within the module where it was declared.

When To Use Local Values

Local values can be helpful to avoid repeating the same values or expressions multiple times in a configuration, but if overused they can also make a configuration hard to read by future maintainers by hiding the actual values used.

Use local values only in moderation, in situations where a single value or result is used in many places and that value is likely to be changed in future. The ability to easily change the value in a central place is the key advantage of local values.