Terraform - why can't I assign a value from 1 variable to another variable

user_13711468 0
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I am trying to assign an output variable from a module to a local variable so that I can conveniently use a local variable. Is there another way ? 

variable "vpc_id" {
  default = "${module.vpc.vpc_id}"
}

Error I am getting is : 

Error: Unsupported argument

  on main.tf line 22, in variable "subnetid_private":
  22:   default = "${module.vpc.subnet_private}"

Variables may not be used here..

I spent good amount of time to google this but could not see any example. Am I missing something here. This is a pretty standard convenience feature of any language.

terraform amazon-web-services
3 Answers
user_4982560

I had the same thought when I first started using Terraform.

The problem is naming. A Terraform variable block is more like a final or constant constructor or method parameter in other languages.

From the Local Value documentation, it says this:

A local value assigns a name to an expression, allowing it to be used multiple times within a module without repeating it.

Comparing modules to functions in a traditional programming language: if input variables are analogous to function arguments and outputs values are analogous to function return values, then local values are comparable to a function's local temporary symbols.

When you write:

variable "vpc_id" {
}

Terraform says "ah, you'd like callers of this module to be able to hand a string in called vpc_id". Definitely not what you are looking for.

For the kind of case you have, a Local Value is what Terraform provides:

locals {
  vpc_id = module.vpc.vpc_id
}
user_1545270

In terraform 0.12, you can use variables in variables like so:

variable "var1" {
  type = string
  default = "this is var 1"
}

variable "var2" {
  type = string
  default = "$${variable.var1}"
}

output of this is:

$ terraform apply
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
user_9422060
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