terranix vs. HCL

Last updated 2026-05-30 · 3 min read

HCL is the language of Terraform. It has its flaws. That is why terranix was born.

This page explains some general differences between the two.

Declarations #

In HCL you would do something like this:

resource "aws_instance" "web" {
  ami           = "${data.aws_ami.ubuntu.id}"
  instance_type = "t2.micro"
  tags = {
    Name = "HelloWorld"
  }
}

Which is the equivalent for the following in terranix:

resource."aws_instance"."web" = {
  ami = "\${data.aws_ami.ubuntu.id}";
  instance_type = "t2.micro";
  tags = {
    Name = "HelloWorld";
  };
}

So while HCL has declarations, Nix only has attribute sets.

References #

In HCL you can only reference variable outputs.

But in terranix, because it is Nix, you can basically reference anything.

For example if you have a resource and you want to reuse its parameters:

resource.hcloud_server.myserver = {
  name = "node1";
  image = "debian-9";
  server_type = "cx11";
};

You can reference parameters the terraform way.

resource.hcloud_server.myotherserver = {
  name = "node2";
  image = "\${ hcloud_server.myserver.image }";
  server_type = "\${ hcloud_server.myserver.server_type }";
};

Or the terranix way:

{ config, ... }:
resource.hcloud_server.myotherotherserver = {
  name = "node3";
  image = config.resource.hcloud_server.myserver.image;
  server_type = config.resource.hcloud_server.myserver.server_type;
};

Or the terranix pro way:

{ config, ... }:
resource.hcloud_server.myotherotherotherserver = {
  name = "node4";
  inherit (config.resource.hlcoud_server) image server_type;
};

terranix references being evaluated when generating the json file. terraform references being calculated when running terraform.

multi line strings #

In terraform you can create multi line strings using the heredoc style.

variable "multiline" {
  description = <<EOT
Description for the multi line variable.
The indentation here is not wrong.
The terminating word must be on a new line without any indentation.
EOT
}

This won’t work in terranix. In terranix you have to use the nix way of multi line strings.

variable.multiline.description = ''
  Description for the multi line variable.
  The indentation here is not wrong.
  All spaces in front of the text block will be removed by terranix.
'';

escaping expressions #

The form ${expression} is used by terranix and terraform. So if you want to use a terraform expression in terranix, you have to escape it. Escaping differs for multi and single line strings.

escaping in single line strings #

In a single line strings, you escape via \${expression}. For example :

variable.hcloud_token = {};
provider.hcloud.token = "\${ var.hcloud_token }";

You can avoid escaping $ with the tfRef function:

{ lib, ... }:
variable.hcloud_token = {};
provider.hcloud.token = lib.tfRef "var.hcloud_token";

escaping in multi line strings #

In multi line strings, you escape via ''${expression}. For example :

resource.local_file.sshConfig = {
  filename = "./ssh-config";
  content = ''
    Host ''${ hcloud_server.terranix_test.ipv4_address }
    IdentityFile ./sshkey
  '';
};

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