Empty arrays interpreted as unset variable when reading from .tf.json formatted variable files

[Fitzsimmons]

It seems empty arrays defined in .tf.json files are not interpreted as set variables. I was able to reduce this problem to a very small test case on Terraform v0.7.3.

provision.tf

variable "foo" {
    type = "list"
}

provider "aws" {
    region = "us-east-1"
}

resource "aws_vpc" "vpcs" {
    count = "${length(var.foo)}"
    cidr_block = "${element(var.foo, count.index)}"
}

After terraform planing with an empty variable in an hcl-style config, the plan comes out as empty as expected:

tfvars/variables.tfvars

foo = []

$ terraform plan -var-file=tfvars/variables.tfvars
[snip]
No changes. Infrastructure is up-to-date.
[snip]

However, if I try this with an equivalent json file, I'm prompted for input.

json/variables.tf.json

{ "foo": [] }

$ terraform plan -var-file=json/variables.tf.json
var.foo
  Enter a value: ^C
Error configuring: Error asking for foo: interrupted

This is specifically a problem with empty arrays in json, because if I populate the array with a value, terraform becomes aware that it has been set.

json/variables.tf.json

{ "foo": ["192.168.77.0/24"] }

$ terraform plan -var-file=variables.tf.json
[snip]
+ aws_vpc.vpcs
    cidr_block:                "192.168.77.0/24"
    default_network_acl_id:    "<computed>"
    etc ...

It would be great if empty arrays read from JSON files were actually interpreted as such.

[mitchellh]

Thanks! I have a PR open for the HCL project that will fix this. Once that is merged, I'll also add a failing test for your case here in Terraform (to avoid regressions in this project) and update the HCL vendor.

[ghost]

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