Can SCALE compact encoding be used with `Option<T>`? If so, how exactly?

Question

I can't find anything on how to use the compact encoding of integers with Option<...>. Is this possible?

And to elaborate, if I do something like this:

#[pallet::call]
... {
    fn my_dispatchable(origin: OriginFor<T>, #[pallet::compact] my_option: Option<u128>) -> DispatchResult {
        // ...
    }
}

Then I get the following error:

the trait `From<parity_scale_codec::Compact<std::option::Option<u128>>>` is not implemented for `std::option::Option<u128>`

This sounds like it's not possible to compactly encode options of integers. Why?

Answer 1

This happens because you apply #[pallet::compact] attribute to the type Option<u128>. u128 can be represented as compact. Option<u128> can't though, since it's an option.

In this case, to make it work you will need to remove the #[pallet::compact] attribute and then change the type to Option<Compact<u128>>. Compact is a usual tuple struct and you can extract the value by the reading .0.