Writing Conditionals in OpenTofu

One of the things that comes up a lot when I talk about OpenTofu (formerly Terraform) configs is how to address conditionals. As of version 1.6.0 and any previous version, conditionals in the if/else sense aren't supported in HCL.

Instead, we get to use ternaries. If you're not familiar, ternaries are a statement that looks at the truthiness of a statement and offers two possible outcomes - one if the statement evaluates to true, and the other if the statement evaluates to false:

provider "aws" {
  region = var.env == "prod" ? "us-east-2" : "us-west-2"
}

In this example, if var.env is "prod", then the AWS region will be set to "us-east-2". If var.env is anything else but "prod", then the AWS region will be set to "us-west-2".

You may then be wondering how we can write the equivalent of an else if. This is where things get ugly.

If we consider that the false-case result is the equivalent of else (that is, in the example above, "us-west-2"), then we can accomplish an else if by offering another ternary:

provider "aws" {
  region = var.env == "prod" ? "us-east-2" : var.env == "stage" ? "us-west-2" : "us-west-1"
}

That is not pleasant to look at, and I'd recommend avoiding it if you can. The easiest way to do so is to add more variables:

provider "aws" {
  region = var.region
}

It may seem obvious to some, but I've seen plenty of codebases hinge multiple conditionals on a single variable like the string value of var.env.

When you have multiple environments, each environment can have its own .tfvars file, which can be targetted during plan and apply, e.g.:

$ tofu plan --var-file ./vars/prod.tfvars