Types

The complete type toolkit exported from @blac/core. These utilities let you derive the state, args, deps, and instance shapes of a container class without hand-writing them, and the branded-ID helpers give you nominal InstanceId strings. Every signature on this page is quoted verbatim from source.

For a task-oriented walkthrough of typing blocs (generic parameters, inference, common pitfalls) see TypeScript. This page is the reference: one heading per export, the exact signature, what it does, and a small example.

Where these come from

The extraction and instance utilities live in @blac/core’s types/utilities module; the branded-ID helpers live in types/branded. Both are re-exported from the package root, so you import everything from @blac/core directly.

All examples assume this setup, which is hidden from the rendered snippet but type-checked:

import { Cubit } from '@blac/core';

interface CounterState {
  count: number;
  label: string;
}

class CounterCubit extends Cubit<CounterState> {
  constructor() {
    super({ count: 0, label: 'idle' });
  }
}

Extracting type parameters

A StateContainer (and therefore Cubit) carries three type parameters: S (state), Args (serializable construction/identity data), and Deps (injected non-serializable handles). These utilities pull each one back out of a container class (the constructor, e.g. typeof CounterCubit), so you never have to re-declare a state shape that already lives on the class.

ExtractState

Signature

export type ExtractState<T> =
  T extends StateContainerConstructor<infer S> ? Readonly<S> : never;

Extracts the state type from a container constructor as a Readonly<S>. This is the type you get back from useBloc and from the state getter — read-only, because state is immutable from the outside. Resolves to never if T is not a container constructor.

type S = ExtractState<typeof CounterCubit>;
type S = {
    readonly count: number;
    readonly label: string;
}

ExtractStateMutable

Signature

export type ExtractStateMutable<T> =
  T extends StateContainerConstructor<infer S> ? S : never;

The same as ExtractState, but without the Readonly<> wrapper — the raw S as the class declared it. Reach for this only when you genuinely need a mutable view (for example, building the next state object before you emit it); prefer ExtractState everywhere a value is being read.

type Draft = ExtractStateMutable<typeof CounterCubit>;
type Draft = CounterState

ExtractArgs

Signature

export type ExtractArgs<T> = T extends new () => StateContainer<
  any,
  infer A,
  any
>
  ? A
  : void;

Extracts the Args type — the serializable data a bloc is constructed/identified with (see Passing Inputs). Falls back to void when the class declares no args. Note the match is against a zero-argument constructor shape (new () =>), which is how container subclasses are written.

interface UserState {
  name: string;
}
class UserCubit extends Cubit<UserState, { userId: string }> {
  constructor() {
    super({ name: '' });
  }
}

type A = ExtractArgs<typeof UserCubit>;
type A = {
    userId: string;
}

ExtractDeps

Signature

export type ExtractDeps<T> = T extends new () => StateContainer<
  any,
  any,
  infer D
>
  ? D
  : Record<string, never>;

Extracts the Deps type — the injected non-serializable handles (clients, services, other blocs) a container depends on. Falls back to Record<string, never> (the “no deps” shape) when none are declared.

interface FeedState {
  posts: string[];
}
class FeedCubit extends Cubit<FeedState, void, { api: { fetch(): void } }> {
  constructor() {
    super({ posts: [] });
  }
}

type D = ExtractDeps<typeof FeedCubit>;
type D = {
    api: {
        fetch(): void;
    };
}

Instance and constructor types

These describe the instance a container class produces, and the constructor shape that @blac/core and @blac/react use to type their entry points.

InstanceReadonlyState

Signature

export type InstanceReadonlyState<T extends StateContainerConstructor = any> =
  Omit<InstanceType<T>, 'state'> & { state: ExtractState<T> };

The instance type of a container class with its state property narrowed to the Readonly state for that class. Useful when you want the full instance (methods, getters) but with a precisely-typed, read-only state.

type ReadonlyInstance = InstanceReadonlyState<typeof CounterCubit>;
declare const c: ReadonlyInstance;
c.state.count;
CounterState.count: number

InstanceState

Signature

export type InstanceState<T extends StateContainerConstructor = any> = Omit<
  InstanceType<T>,
  'state'
> & { state: ExtractStateMutable<T> };

Like InstanceReadonlyState, but the state property is the mutable S rather than Readonly<S>. Reach for it only when you specifically need a writable view of state on a typed instance.

type MutableInstance = InstanceState<typeof CounterCubit>;

StateContainerInstance

Signature

export type StateContainerInstance<S extends object = any> = Omit<
  StateContainer<S, any, any>,
  'state'
> & { state: Readonly<S> };

A StateContainer instance keyed by its state type S rather than by a concrete class, with state narrowed to Readonly<S>. Use it when you want to describe “any container holding this state shape” without pinning a specific subclass.

interface CounterState {
  count: number;
  label: string;
}
function readCount(c: StateContainerInstance<CounterState>) {
  return c.state.count;
}

StateContainerConstructor

Signature

export type StateContainerConstructor<S extends object = any> = new (
  ...args: any[]
) => StateContainer<S, any, any>;

The minimal constructor type for a container class, parameterized by state S. It is the constraint the extraction utilities (ExtractState, ExtractStateMutable, InstanceReadonlyState, InstanceState) match against. Use it where you only care that something is a container class.

const Ctor: StateContainerConstructor<CounterState> = CounterCubit;

Branded IDs

BlaC tags instance-identity strings with a brand so a plain string cannot be passed where an InstanceId is expected. The brand is a compile-time-only phantom property keyed by a unique symbol; it has no runtime footprint — branded values are just strings at runtime.

Brand

Signature

declare const brand: unique symbol;

export type Brand<T, B> = T & { [brand]: B };

The general nominal-typing helper: intersects a base type T with a phantom property under a private unique symbol, tagged by the brand identifier B. Two Brands with different B are incompatible even when T is identical, which is what prevents accidental mixing of similar primitive types.

type UserId = Brand<string, 'UserId'>;
type OrderId = Brand<string, 'OrderId'>;

declare const u: UserId;
declare function loadOrder(id: OrderId): void;
loadOrder(u);
Error ts(2345)  ― Argument of type 'UserId' is not assignable to parameter of type 'OrderId'.
  Type 'UserId' is not assignable to type '{ [brand]: "OrderId"; }'.
    Types of property '[brand]' are incompatible.
      Type '"UserId"' is not assignable to type '"OrderId"'.

BrandedId

Signature

export type BrandedId<B> = Brand<string, B>;

A convenience alias for the common case of branding a string: BrandedId<B> is exactly Brand<string, B>. Use it whenever the base type is a string ID.

type SessionId = BrandedId<'SessionId'>;

InstanceId

Signature

export type InstanceId = Brand<string, 'InstanceId'>;

The branded string type BlaC uses for state-container instance identities — BrandedId<'InstanceId'> by another name. Registry and identity APIs accept and return this rather than a bare string, so an arbitrary string cannot stand in for an instance key by accident.

declare function lookup(id: InstanceId): void;
lookup('not-branded');
Error ts(2345)  ― Argument of type 'string' is not assignable to parameter of type 'InstanceId'.
  Type 'string' is not assignable to type '{ [brand]: "InstanceId"; }'.

instanceId()

Signature

export function instanceId(id: string): InstanceId {
  return id as InstanceId;
}

The value-level helper that brands a plain string as an InstanceId. It is a pure cast at runtime (returns the input unchanged); its only job is to give you a typed InstanceId to hand to APIs that require one, without writing the as InstanceId assertion yourself.

const id: InstanceId = instanceId('user-42');

See also

• TypeScript — typing blocs end to end: generics, inference, and pitfalls
• Cubit — the class these utilities extract types from
• Instance Management — acquire / borrow / release and the registry surface
• Passing Inputs — the args / deps identity model that ExtractArgs and ExtractDeps read
• useBloc — where ExtractState shows up as the hook’s return type