Terraform workspaces with locals

locals.tf

locals {
  vpc_type = "${terraform.workspace == "prod" ? "prod" : "dev"}"
  vpc_id = "${local.vpc_type == "prod" ? var.vpc_ids["prod"] : var.vpc_ids["dev"]}"
}

network.tf

resource "aws_subnet" "ext_subnets" {
  vpc_id     = "${local.vpc_id}"
  cidr_block = "${var.ext_subnet_cidr[local.vpc_type]}"

  tags {
    Name = "EXT_1_${terraform.workspace}"
  }
}

terraform.tfvars

vpc_ids = {
  prod = "vpc-aad01dd1"
  dev = "vpc-f78f568c"
}

ext_subnet_cidr = {
  prod = "10.0.0.0/24"
  dev = "10.1.0.0/24"
}