Unable to use output variable as list

[hashibot]

This issue was originally opened by @nusnewob as hashicorp/terraform#8048. It was migrated here as part of the provider split. The original body of the issue is below.

---

Terraform Version

Terraform v0.7.0

Affected Resource(s)

• aws_elb
• aws_db_subnet_group

Terraform Configuration Files

main.tf

module "subnets" {
  source = "./modules/test"
}

resource "aws_elb" "elb" {
  name = "test"
  subnets = ["${module.subnets.subnets["public"]}"]
  listener {
    lb_port = 80
    lb_protocol = "http"
    instance_port = 80
    instance_protocol = "http"
  }
}

or

...
  subnets = ["${split(",", join(",", module.subnets.subnets["public"]))}"]
...

modules/test/module.tf

data "null_data_source" "subnets" {
  inputs = {
    public = ["subnet-abcd1234","subnet-abcd1235","subnet-abcd1236"]
  }
}

output "subnets" {
  value = "${data.null_data_source.subnets.input}"
}

Debug Output

https://gist.github.com/nusnewob/b3230d0d83c40917516a068b3532092f

Expected Behavior

Should be able to pass the module output as list variable

Actual Behavior

Error refreshing state: 1 error(s) occurred:

* inputs: 1 error(s) decoding:

* '[public]' expected type 'string', got unconvertible type '[]interface {}'

Steps to Reproduce

1. terraform apply or terraform plan

[mattlqx]

There are many scenarios where a null_data_source output would be useful as a list rather than a simple string. Is there are specific technical reason to not allow this?

My specific use-case is taking a comma-delimited value from Consul (via consul_keys, which is all string-based) and converting it to a list with a null_data_source so I would only have to deserialize once rather than every place where that value is used.

I suppose I could write a module to do this process for me, but that seems overkill.

[in4mer]

You can't do this because it's a list inside of a hash (inputs). There's a core TF bug around improper or incomplete nested type passing.

Do this instead:

data "null_data_source" "stringified" {
  inputs = {
    public = "subnet-1234,subnet-3456,subnet-5678"
  }
}

output "subnets" {
  value = "${split(",", data.null_data_source.stringified.inputs.public)}"
}

[kunupat]

@in4mer This gives self reference not allowed error.

[bflad]

Terraform 1.4 will contain a new terraform_data resource which can accept input data of any type, reflections of that data into an unknown output of the same type, and value updates. Terraform prereleases are available if anyone wants to try out the upcoming functionality.

Reference: https://github.com/hashicorp/terraform/blob/main/website/docs/language/resources/terraform-data.mdx

[bflad]

Terraform 1.4.0 has been released today with the new terraform_data resource. It should not impose any type restrictions on input or output. If there are any feature requests or bug reports with the terraform_data resource, please create an issue in the Terraform core issue tracker.

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