example-usage.ts

class Maybe<A> {
 [Generic.repr]: Generic<MaybeRepr, A>
}
interface MaybeRepr extends Repr {
  type: Maybe<this[1]>
}
type test1 = Of<MaybeRepr, string> // Maybe<string>
  
class Result<T, E> {
  [Generic.repr]: Generic<ResultRepr, T, E>
}
interface ResultRepr extends Repr {
  type: Result<this[1], this[2]>
}
type test2 = Of<ResultRepr, number, string> // Result<number, string>

tshkt.ts

/* Original repository: https://github.com/strax/tshkt */

export interface Repr {
	1: unknown
	2: unknown
	3: unknown
	4: unknown
	type: unknown
}

export type Generic<
	TRepr,
	A1,
	A2 = unknown,
	A3 = unknown,
	A4 = unknown
> = TRepr & TypeArgument<A1, A2, A3, A4>

export namespace Generic {
	export declare const repr: unique symbol
}

export interface TypeArgument<
	T1 = unknown,
	T2 = unknown,
	T3 = unknown,
	T4 = unknown
> {
	1: T1
	2: T2
	3: T3
	4: T4
}

export interface HasGeneric<T, A1, A2 = unknown, A3 = unknown, A4 = unknown> {
	[Generic.repr]: Generic<T, A1, A2, A3, A4>
}

export type Of<T, A1, A2 = unknown, A3 = unknown, A4 = unknown> = [T] extends [Repr]
	? Generic<T, A1, A2, A3, A4>['type']
	: HasGeneric<T, A1, A2, A3, A4>