Can I tell the parser more about an expected node argument?

[theophile-scrive]

Hi @brandonchinn178!

I have the following KDL format:

inventory "base" {
	var "name" {
		value "value"
	}
}

And I want to improve the error reporting in case the inventory name name is omitted:

inventory {
	var "name" {
		value "value"
	}
}

At the moment, a rendered error gives me the following:

        At: inventory #0
          Expected arg #0

I'd like to be able to report an error saying Node inventory is expecting an argument "name" of type "text".

The parser looks like this at the moment:

KDL.nodeWith "inventory" $ do
  name <- KDL.arg @Text

I guess I could use optional and throw a custom error if I see a Nothing?

---

Note that I adopt this syntax because the tool I'm building is aimed at people who are exposed to Terraform, which has HCL configuration such as:

module "outscale-vm" {
  source       = "../modules/outscale-vm"
  vm_type      = "tinav6.c2r4p1"
  keypair_name = "bastion"
}

but if this is stretching the language / parser beyond what it's been made for, I can understand that I should use props instead.

[brandonchinn178]

Right now, you could use KDL.fail to create custom messages. But there's certainly room for improvement with error messages here; we could already add in the type info today, and we could provide a KDL.label combinator to tag things. If you have some time, a PR would be welcome! But KDL.fail should be able to unblock you right now

[theophile-scrive]

@brandonchinn178 Do you have a pre-existing example of a label function in mind? Could I take inspiration from, say, Megaparsec?

[brandonchinn178]

Yep, just like megaparsec. In terms of implementation, you'd probably add a new Context on the stack if an error occurs containing arg name/type? Not sure what the best approach is here

[theophile-scrive]

@brandonchinn178 Would you approve if I explored adding a label parameter to argWith'? So that we can have it in its addContext?

We can also have a label function to wrap a parser but I'm unsure how to detect that a parser has succeeded (and how to inject a context in it, in case of failure).

[brandonchinn178]

No, I don't think we should add label to argWith'. The idea is to create a new ContextLabel constructor and in label, always add the ContextLabel. Then on failure, we would see [ContextLabel, ContextArg] and show an appropriate error

[theophile-scrive]

Ah, got it. :)