COMPONENT: Add support for optional objects in variable definitions with non-trivial default values

[zoonage]

Description

I'd like to be able to define a variable like this in cdktf, where a is optional but b is not.

variable "complex" {
  type = object({
    a = optional(number, 1)
    b = string
  })
  default = {}
}

Background context: I'm looking into cdktf adoption to try and simplify a large existing Terraform codebase.

The first area I'm looking at is removing duplicated complex variable type signatures. These can be up to ~30 lines that we manually keep in sync to work around this issue.

I'm planning on doing this by synthesising a bit of the module (such as specific variable definitions) by using a shared library but keeping the rest of the existing Terraform in place.

Happy to try and implement this

References

No response

Help Wanted

• I'm interested in contributing a fix myself

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[zoonage]

Happy to have a shot at implementing this. The only question I have is I'm not sure if there is a way to build a complex type for a default like

{
  otel-agent = { image = "quay.io/zoonage/otel-agent:latest" }
  some-sidecar = { image = "quay.io/zoonage/otel-agent:latest", resources = { cpu = 1 } }
}

or if that's some extra functionality that'll be needed for this

[chrcaleb]

I am extremely interested in this feature as well. I am hoping by providing the example Hashi provides for HCL here, the complexity is less than you think.

The optional modifier takes one or two arguments.

Type: (Required) The first argument specifies the type of the attribute.
Default: (Optional) The second argument defines the default value that Terraform should use if the attribute is not present. This must be compatible with the attribute type. If not specified, Terraform uses a null value of the appropriate type as the default.

variable "buckets" {
  type = list(object({
    name    = string
    enabled = optional(bool, true)
    website = optional(object({
      index_document = optional(string, "index.html")
      error_document = optional(string, "error.html")
      routing_rules  = optional(string)
    }), {})
  }))
}

[jsteinich]

I don't think it would be too difficult to add a simple version of this. If you look at https://github.com/hashicorp/terraform-cdk/blob/main/packages/cdktf/lib/terraform-variable.ts there are some static functions meant for building complex types. A new method for optional could be created as a quick version. Building better default support would take some more thought.

Actually using complex variables effectively within cdktf is another issue, but having basic support may help make things easier.