map variables should support interpolation

[albertsj1]

I hit this problem while trying to find a solution to using the count parameter to create 3 subnets and then use those 3 subnets for 3 new instances. Mitchell suggested I use a map variable and then a lookup to get the subnet_id. Here's the code I tried.

resource "aws_subnet" "public" {
  vpc_id = "${aws_vpc.default.id}"

  cidr_block = "${concat(lookup(var.vpc_cidr_base, var.aws_region), ".", count.index ,".0/24")}"
  availability_zone = "${lookup(concat("aws_region_", var.aws_region), concat("key", count.index))}"
  count = 3
}

variable "public_ids" {
  default = {
    key0 = "${aws_subnet.public.0.id}"
    key1 = "${aws_subnet.public.1.id}"
    key2 = "${aws_subnet.public.2.id}"
  }
}

resource "aws_instance" "nat" {
  ami = "${lookup(var.aws_nat_amis, var.aws_region)}"
  availability_zone = "${lookup(concat("aws_region_", var.aws_region), concat("key", count.index))}"
  instance_type = "t2.micro"
  key_name = "${var.aws_key_name}"
  security_groups = ["${aws_security_group.nat.id}"]
  subnet_id = "${lookup(var.public_ids, concat("key", count.index))}"
  associate_public_ip_address = true
  source_dest_check = false
  count = 3
  depends_on = ["aws_subnet.public", "aws_security_group.nat"]
}

The error I received when running 'terraform plan' was:

• Variable 'public_ids': cannot contain interpolations

It was suggested by @mitchellh that I use a variable map to get to the newly created subnet_ids via the lookup function. Is there another way to do this that I'm not seeing?

Besides this not working, this is a really verbose and messy solution. I initially tried to use just a number for the key name, but terraform gave me an error. It seems like key names must start with a letter. This required me to use 'concat' to specify the correct key name for the lookup.
Ideally, I wish I was able to simply define in the aws_instance resource declaration the following:

subnet_id = "${aws_subnet.public.${count.index}.id}"

This would require recursive string interpolation to be added though.

[mitchellh]

Ah, this isn't quite what I meant. I think there is a better solution to this problem. We're not very interested at this time to support interpolations within variables since then variables also need to be a separate phase of the dependency graph and we don't want to do that yet.

There is another issue (tagged "docs") tracking this. We'll come up with some solution there.

[mrud]

Just wondering what the suggested solution for this is? I am running into the same issue

[tamsky]

Can someone please link to the issue mentioned by mitchellh:

There is another issue (tagged "docs") tracking this. We'll come up with some solution there.

I've spent at least 15 minutes looking and have been unable to find a related one having label:documentation.

[apparentlymart]

I don't know which one @mitchellh was talking about but this looks a lot like #4084 to me.

I posted a workaround there of using a module as a place to stash some data, but I don't think that workaround would work for storing a map as shown in this issue, since modules can only export flat strings.

[fjromerom]

I have suffered same problem :(

[robert-chiniquy]

So no insights on this yet? It got me too.

[albertsj1]

While recursive interpolation and interpolation within variables are still not features, I find the interpolation methods that are available now work well enough to get the job done.
For example, instead of this:

subnet_id = "${aws_subnet.public.${count.index}.id}"

do this:

subnet_id = "${element(aws_subnet.public.*.id, count.index)}"

[ghost]

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