TS: Higher Kinded Types

hkt.ts

// Higher Kinded base type
// inspired by 
// https://github.com/arktypeio/arktype/blob/main/ark/util/hkt.ts
// and
// https://github.com/gvergnaud/hotscript/blob/main/src/internals/core/Core.ts

declare const rawArgs: unique symbol;
type rawArgs = typeof rawArgs;

interface Hkt {
  [rawArgs]: unknown;
  args: this[rawArgs] extends infer args extends unknown[] ? args : never;
  arg0: this[rawArgs] extends [infer arg, ...any] ? arg : never;
  arg1: this[rawArgs] extends [any, infer arg, ...any] ? arg : never;
  return: unknown;
}

type Apply<fn extends Hkt, args> = (fn & {
  [rawArgs]: args;
})["return"];

/** example: Capitalize a string */
interface Cap extends Hkt {
	return: string extends this["arg0"]
	  ? `${Uppercase<string>}${string}`
	  : this["arg0"] extends `${infer C}${infer Rest}`
	  ? `${Uppercase<C>}${Rest}`
	  : never;
  }