ts-function.ts

// Creates functions that can be run either at compile time or run time.

export abstract class Function<I> {
    declare input: I
    declare output: ReturnType<this["x"]>

    // @ts-ignore
    abstract x(input: this["input"])
}

export function createFunction<F extends Function<any>>(
    Fn: new () => F,
): TSFunction<F> {
    const { x } = new Fn()
    return ((input: F["input"]): F["output"] => x(input)) as any
}

declare const tsFunctionMarker: unique symbol

export type TSFunction<F extends Function<any>> = {
    <T extends F["input"]>(
        input: T,
    ): F["output"] & ReturnType<(F & { input: T })["x"]>

    [tsFunctionMarker]: F
}

export type Call<
    F extends TSFunction<Function<any>>,
    I extends F[typeof tsFunctionMarker]["input"],
> = F[typeof tsFunctionMarker]["output"]
    & ReturnType<(F[typeof tsFunctionMarker] & { input: I })["x"]>