Terraform: How Do I Setup a Resource Based on Configuration - conditional-statements

Question

So here is what I want as a module in Pseudo Code:
IF UseCustom, Create AWS Launch Config With One Custom EBS Device and One Generic EBS Device
ELSE Create AWS Launch Config With One Generic EBS Device
I am aware that I can use the 'count' function within a resource to decide whether it is created or not... So I currently have:
resource aws_launch_configuration "basic_launch_config" {
count = var.boolean ? 0 : 1
blah
}
resource aws_launch_configuration "custom_launch_config" {
count = var.boolean ? 1 : 0
blah
blah
}
Which is great, now it creates the right Launch configuration based on my 'boolean' variable... But in order to then create the AutoScalingGroup using that Launch Configuration, I need the Launch Configuration Name. I know what you're thinking, just output it and grab it, you moron! Well of course I'm outputting it:
output "name" {
description = "The Name of the Default Launch Configuration"
value = aws_launch_configuration.basic_launch_config.*.name
}
output "name" {
description = "The Name of the Custom Launch Configuration"
value = aws_launch_configuration.custom_launch_config.*.name
}
But how the heck do I know from the higher area that I'm calling the module that creates the Launch Configuration and Then the Auto Scaling Group which output to use for passing into the ASG???
Is there a different way to grab the value I want that I'm overlooking? I'm new to Terraform and the whole no real conditional thing is really throwing me for a loop.

Answer 1

Terraform: How to conditionally assign an EBS volume to an ECS Cluster
This seemed to be the cleanest way I could find, using a ternary operator:
output "name {
description = "The Name of the Launch Configuration"
value = "${(var.booleanVar) == 0 ? aws_launch_configuration.default_launch_config.*.name : aws_launch_configuration.custom_launch_config.*.name}
}
Let me know if there is a better way!

Answer 2

You can use the same variable you used to decide which resource to enable to select the appropriate result:
output "name" {
value = var.boolean ? aws_launch_configuration.custom_launch_config[0].name : aws_launch_configuration.basic_launch_config[0].name
}
Another option, which is a little more terse but arguably also a little less clear to a future reader, is to exploit the fact that you will always have one list of zero elements and one list with one elements, like this:
output "name" {
value = concat(
aws_launch_configuration.basic_launch_config[*].name,
aws_launch_configuration.custom_launch_config[*].name,
)[0]
}
Concatenating these two lists will always produce a single-item list due to how the count expressions are written, and so we can use [0] to take that single item and return it.