is-safe_type_guard.ts

/**
 * Stricter and Safer version of Type Guards.
 *
 * It takes function that actually returns Output type and constructs
 * type guard which is actually safe and can't ever go out of sync.
 * for example this type guard compiles:
 * ```ts
 * function isNumber(x: any): x is number {
 *   return typeof x === "string";
 * }
 * ```
 *
 * While using `is` you can't write such gibberish.
 * ```ts
 * is(function isNumber(x: unknown) {
 *   return typeof x === "number" ? x : undefined;
 * }) // : (value: unknown) => value is number
 * ```
 */
export function is<Input, Output extends Input, Args extends unknown[]>(
  f: (value: Input, ...args: Args) => Output | undefined
) {
  return (value: Input, ...args: Args): value is Output => {
    if (f(value, ...args) !== undefined) {
      return true;
    }
    {
      return false;
    }
  };
}