Terraform won't accept variable default interpolation or handle layered interpolations

Question

I have a module that creates multiple outputs. The value of each output being an account number.

I'd like to use the count parameter on a resource to do iteration using the values from the module mentioned above. However, I'm learning that you cannot do interpolation on a variable default or layered interpolation.

What is the right way to handle this in terraform?

variable "service_node_accounts" {
  description = "List of Account IDs"
  type        = "list"
  default     = ["${module.accounts.qa}", "${module.accounts.staging}", "${module.accounts.prod}"]
}

data "aws_ami" "service_node_1_0" {
    filter {
        name   = "name"
        values = ["service-node-1.0"]
    }
    owners = ["self"] # Canonical
}

resource "aws_ami_launch_permission" "service_node_1_0" {
  count      = "${length(var.service_node_accounts)}"
  image_id   = "${aws_ami.service_node_1_0.id}"
  account_id = "${var.service_node_accounts[count.index]}"
}

terraform plan...
default may not contain interpolations

Answer 1

Consider using locals where interpolations are allowed.

locals {
  # Untested but should work in theory
  service_node_accounts = ["${module.accounts.qa}", "${module.accounts.staging}", "${module.accounts.prod}"]
}