How to declare an element with an argument sink

[Mc-Zen]

Is it possible to declare an element that has a sink of (mostly positional I guess) arguments, like grid or stack?

[PgBiel]

Good question! While allow-unknown-fields is available for unlimited named arguments, that's not really doing the job of a sink, let alone for positional arguments. Instead, you will have to override the arg parser and/or the constructor to accept arguments that don't belong to fields.

I think the constructor is easier to override if we assume that the positional arguments are required, so they can't be specified in set rules. However, if they are not required, then overriding parse-args is needed so they are recognized in set rules.

If you need to override parse-args and don't want to do everything from scratch, there is a parsing module exported by Elembic (not yet mentioned in the docs) which contains two functions, parse-fields (convert array of field() into a specialized dictionary with fields) and generate-arg-parser(fields dictionary, typecheck: true, ...) - you can use this to generate the fields dictionary before declaring the element, and then regenerate the original arg parser, which you can then use inside your parse-args override.

(I think I should probably just make parse-args be a function original => args => ..., haha)

[PgBiel]

However, this is expected to become a built-in feature in the future.

[Mc-Zen]

Alright, cool. Regarding things like grid, where only positional arguments are wanted (which is my use case and which also covers all there is in the Typst standard library), overriding the constructor should do the trick.

[PgBiel]

I've updated parse-args to receive the default argument parser.

There are now instructions for this exact request in the docs: https://pgbiel.github.io/elembic/elements/creating/overriding-constr.html#argument-sink

[Mc-Zen]

Great, it works. However, now "set" rules are broken for that element afaics.

[Mc-Zen]

When I do

#show: e.set_(my-element, some-named-field: 2)

it complains with this part:

      assert(false, message: "element 'sunk': unexpected positional arguments\n  hint: these can only be passed to the constructor")

[PgBiel]

Oh, that was a terrible oversight on my part. I forgot to check whether the positional arguments weren't empty before throwing the error, and if they were, not passing the additional argument.

I've fixed the example in the docs now. Thanks!

[Mc-Zen]

Now it works perfectly