Ability to define "branded" types.

[indietyp]

I am currently searching for a way to map TypeScript branded types, like https://github.com/kourge/ts-brand cleanly using specta. My current (flawed) approach is:

#[cfg_attr(feature = "wasm", derive(specta::Type))]
struct Brand<T, const MARKER: &'static str> {
    _marker: core::marker::PhantomData<T>,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
#[cfg_attr(feature = "wasm", derive(specta::Type))]
#[repr(transparent)]
pub struct AccountId(
    #[cfg_attr(feature = "wasm", specta(type = Brand<Uuid, "AccountId">))]
    Uuid,
);

Which reduces to:

export type AccountId = Brand<string>
export type AddAccountGroupMember = { accountGroupId: AccountGroupId; accountId: AccountId }
export type Brand<T> = { _marker: null }

Now, this is not ideal due to several factors:

1. need to remove export Type Brand<T>
2. need to manually add import header for import {Brand} from ts-brand; (not that bad)
3. needs nightly support for adt_const_params, which is incomplete.
4. The const parameter is not carried over.

What I would love (in a perfect world) would be something similar to:

import {Brand} from 'ts-brand';

export type Uuid = Brand<Uuid, 'Uuid'>
export type AccountId = Brand<Uuid, 'AccountId'>
export type AddAccountGroupMember = { accountGroupId: AccountGroupId; accountId: AccountId }

How would I approach this problem? Any help would be greatly appreciated!

[oscartbeaumont]

The const parameter is not carried over.

Tracked in #17. I intend to support this in Specta v2 (mostly to avoid the need for Specta v3) but it's worth noting Serde doesn't support const parameters.

need to remove export Type Brand

It's hacky but this is possible (and sadly is something we do in Spacedrive).

If you're using 2.0.0-rc.x releases you will need to use main for (cab26b1) or if your using the crates.io releases the TypeMap (previously TypeDefs) was just a BTreeMap so you can do whatever.

Getting access to the TypeMap it's not super easy right now, you would need to copy the implementation of either specta::ts::export or specta::ts::inline into your own crate based on which one you are using.

This is the sorta feature that probably needs its own higher-level abstraction outside of the specta core crate.

need to manually add import header for import {Brand} from ts-brand; (not that bad)

Right now you would have to do this manually after Specta runs. Tauri Specta has a header propery which is set by the user and is injected at the start of the file. This is something Specta should probs have too which was an oversight.

I created #202 to track this

How would I approach this problem?

I'm honestly not sure we have a way to express this at the moment. It could be done with #110 or #17 but neither are supported at the moment and both have unanswered questions about exactly how I would want them to work.

[oscartbeaumont]

A correction to the bit about the header property only matters when using the export feature flag. The normal specta::ts::* stuff doesn't suffer that problem because it's on you to deal with the file writing.