Was raised here too, but not addressed - the author decided to close the ticket :(

#609

Hey @khos2ow , sorry for the ping. This is still relevant for my use-case.

Could you please explain the logic behind null -> "" in the HCL tfvars?

In Terraform/OpenTofu, nil/null simply means unknown, or more often, not-yet-known. Being "unknown" is a valid state.

In Go, the language is strongly typed, meaning that great lengths are taken to avoid nil. Terraform's interpretation of null is different from Go's interpretation of nil. The "zero type" (variable initialized with a type, but without a value) of a string is "". It is (a) definitely a string (b) with no value.

• https://go.dev/tour/basics/12
• https://go.dev/play/p/dO_Ro-QE9ky

Keeping nil or null as a sustained value in low-level Terraform required an entirely-new custom type. See Unknown Values and the Dynamic Pseudo-Type for an under-the-hood explanation.

I haven't looked closely enough at the internals of this project (yet) to get a sense of how complex it might be to add support for this, but I've used the hclparse package to do some custom HCL parsing and generation, and it's not for the faint of heart. You are perfectly correct that Terraform interprets the code this way, but a separate tool (e.g., this one) would need to do a non-trivial amount of work to interpret the same value the same way.

My speculation is that, up to this point, the "juice hasn't been worth the squeeze."

Anyway, that's the explanation for how it works in Go, and my unsurprise at null being translated to "" in this project.

Thanks for the very elaborate explanation! TIL :)

I went in the code and tried to unwrap if there's any special magic done for typing and stumbled upon this: https://github.com/terraform-docs/terraform-docs/blob/master/format/tfvars_hcl.go#L49-L54

If I read it correctly - this condition is specifically handling the case and changing null to "" for whatever reason 😅

I'll try to test this w/o the condition to see if it works ok.

UPD: This is it! Worked like a charm. I'll be making a PR to address that, though still not clear what was the original intent behind null -> "" in this tool.