A hacky way to create a dynamic list of maps in Terraform

main.tf

# The goal: create a list of maps of subnet mappings so we don't have to statically hard-code them in aws_lb
# https://www.terraform.io/docs/providers/aws/r/lb.html#subnet_mapping
locals {
  # These represent dynamic data we fetch from somewhere, such as subnet IDs and EIPs from a VPC module
  subnet_ids = ["subnet-1", "subnet-2", "subnet-3"]
  eips       = ["eip-1", "eip-2", "eip-3"]
}

# Here's the hack! The null_resource has a map called triggers that we can set to arbitrary values. 
# We can also use count to create a list of null_resources. By accessing the triggers map inside of
# that list, we get our list of maps! See the output variable below.
resource "null_resource" "subnet_mappings" {
  count = "${length(local.subnet_ids)}"

  triggers {
    subnet_id     = "${element(local.subnet_ids, count.index)}"
    allocation_id = "${element(local.eips, count.index)}"
  }
}

# And here's the result! We have a dynamic list of maps. I'm just outputting it here, but we should
# be able to take the same value and set it as the input to aws_lb's subnet_mapping param.
output "subnet_mappings" {
  value = "${null_resource.subnet_mappings.*.triggers}"
}