Concat on two values assigned by cidrsubnet?

Question

I am trying to concat two lists of subnets into module.vpc.public_subnets

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"
  ...
  public_subnets = [
    concat(local.ext_a_subnets,local.ext_b_subnets)
  ]
  ...
}
...
locals {
  ...
  ext_a_subnets = [
    for num in range(length(local.azs)) :
    cidrsubnet(local.cidr, 8, num + 10)
  ]
  ext_b_subnets = [
    for num in range(length(local.azs)) :
    cidrsubnet(local.cidr, 8, num)
  ]
  ...
}

When I run terraform validate, I get the below error:

Error: Invalid value for module argument

  on main.tf line 45, in module "vpc":
  45:   public_subnets = [
  46:     concat(local.ext_a_subnets,local.ext_b_subnets)
  47:   ]

The given value is not suitable for child module variable "public_subnets"
defined at .terraform/modules/vpc/variables.tf:151,1-26: element 0: string
required.

However, putting the cidrsubnet for loop in module.vpc.public_subnets directly works fine:

  public_subnets = [
    for num in range(length(local.azs)) :
    cidrsubnet(local.cidr, 8, num + 10)
  ]

Is concat not the function I need? Pretty new to terraform so appreciate all the help.

user_248823 · Answer

concat returns a list. So assuming that your concat actually works, it should be:

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"
  ...
  public_subnets = concat(local.ext_a_subnets,local.ext_b_subnets)
  ...
}

Concat on two values assigned by cidrsubnet?

1 Answers