This expression is not callable, Each members of the union

Question

I have aligned my approach with my nearest current architecture, striving to minimize complexity as much as possible.

I have aimed for the closest approximation to a satisfactory result, but I encountered a single failed test with this approach.

I'm on the verge of giving up, as it has already been three days! However, if your expertise can uncover a solution that ensures the success of all tests, it would be truly amazing!

the getComponentFromEntity is maybe the key, maybe need a black magic type pattern to solve this ?!

Thank you for your valuable time

◽playground link

//utils types
type Constructor<T> = { new(...args: any): T };
type ExtractComponentType<T> = T extends Entity<infer C> ? C : never;
type EntitiesCareMap<C extends Component> = Map<number, Entity<C>>
type ComponentType<T extends Component = Component> = Constructor<T>;
type TupleToInstances3<T extends readonly unknown[]> = {
    [K in keyof T]: T[K] extends Constructor<infer U> ? U extends {} ? U : never : never;
}

type ExtractSystemComponents4<
    S extends Rules,
    K extends RULES
> = S[K] extends ComponentType[] ? S[K] extends never[] ? UnknowComponent : TupleToInstances3<S[K]>[number] : never;

interface SystemUpdate<S extends System = System, R extends Rules = S['rules']> {
    entities: EntitiesCareMap<
        ExtractSystemComponents4<R, RULES.hasAll>
    >;

}
// issue

class Entity<
    C extends Component = Component,
> {
    declare public components: Set<C>;

    get<T extends C>(componentClass: Constructor<T>): T {
        return undefined as unknown as T;
    }
        has<T extends Component>( componentClass: Constructor<T> ): this is Entity<T> {
        return false;
    }

}
abstract class Component {
    foo() { }
}

enum RULES {
    hasAll,
}
type Rules = { readonly [K in RULES]?: ComponentType[] };
abstract class System {
    abstract rules: Rules;
    abstract onUpdate(t: SystemUpdate<System, Rules>): void;
}

export class UnknowComponent extends Component {
    #component!: never;
}

export class AComponent extends Component {
    #component!: never;
}
export class BComponent extends Component {
    #component!: never;
}
export class CComponent extends Component {
    #component!: never;
}
export class DComponent extends Component {
    #component!: never;
}

class SystemA extends System {
    public rules = {
        [RULES.hasAll]: [AComponent, BComponent],
    };

    onUpdate({entities}: SystemUpdate<SystemA>) {
        entities.forEach(( e ) => {
            e.get(BComponent)// 🟢 this should pass.
            e.get(AComponent)// 🟢 this should pass.
            e.get(CComponent)// 🔴 this should error
            if (e.has(CComponent)) {
                e.get(CComponent)// 🟢 this should pass.
                e.get(DComponent)// 🔴 this should error
                if (e.has(DComponent)) {
                    e.get(DComponent)// 🟢 this should pass.
                }
            }
        });
    }
}



declare const ab: Entity<BComponent> | Entity<BComponent | CComponent>;

/** Get a components from entity */
function getComponentFromEntity<E extends Entity, C extends ExtractComponentType<E>>(entity: E, component: Constructor<C>): C {
    return entity.get(component);
}

getComponentFromEntity(ab, BComponent) // 🟢 this should pass.
getComponentFromEntity(ab, AComponent) // 🔴 this should error.
getComponentFromEntity(ab, CComponent) // 🔴 this should error.
//^?

declare const a: Entity<BComponent | CComponent>;
a.get(BComponent)// 🟢 this should pass.
a.get(AComponent)// 🔴 this should error

Answer 1

I'd say that you want ExtractComponentType<T> to turn unions in T to intersections in the output type. So ExtractComponentType<A | B> will be equivalent to ExtractComponentType<A> & ExtractComponentType<B>. (That is, you want to distribute your operation over unions in T but in a contravariant way (see Difference between Variance, Covariance, Contravariance and Bivariance in TypeScript for more info on variance).

That's because when you call getComponentFromEntity(e, c), if c is of type Entity<A | B> then c can be either A or B (because Entity<A | B> accepts either), but if c is of type Entity<A> | Entity<B> then you don't know which it accepts, so c has to be both A and B for that to be safe.

So let's implement it.

---

Here's one way:

type ExtractComponentType<T> =
    (T extends Entity<infer C> ? ((x: C) => void) : never) extends
    (x: infer I) => void ? I : never;

type X = ExtractComponentType<Entity<BComponent | CComponent>>;
// type X = BComponent | CComponent
type Y = ExtractComponentType<Entity<BComponent> | Entity<CComponent>>;
// type Y = BComponent & CComponent

You can see that it works as intended. The implementation uses a contravariance trick with conditional types, as described in Transform union type to intersection type. Since function types are contravariant in their parameter types, we move the type into a function parameter position before inferring from it.

---

We're 95% of the way there. Here's the rest:

function getComponentFromEntity<
    E extends Entity,
    C extends Component & ExtractComponentType<E>
>(entity: E, component: Constructor<C>): C {
    return entity.get(component);
}

All I had to do there is tell the compiler that C would definitely be a Component of some sort, to prevent the implementation from complaining. TS can't really do higher order reasoning about generic conditional types, so even though ExtractComponentType<E> must be compatible with Component by construction, the compiler fails to see it. So I added Component & to fix that.

---

Let's test it:

declare const ab: Entity<BComponent> | Entity<BComponent | CComponent>;
getComponentFromEntity(ab, BComponent) // 🟢 okay
getComponentFromEntity(ab, AComponent) // 🔴 error!
getComponentFromEntity(ab, CComponent) // 🔴 error!

declare const a: Entity<BComponent | CComponent>;
a.get(BComponent)// 🟢 okay
a.get(AComponent)// 🔴 error!

Looks like the behavior you wanted!

Playground link to code