samchon/typia v5.5.0

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export namespace functional {
  export function assertFunction<T extends Function>(func: T): T;

  export function isFunction<T extends (...args: any[]) => any>(
    func: T,
  ): T extends (...args: infer Arguments) => infer Output
    ? Output extends Promise<infer R>
      ? (...args: Arguments) => Promise<R | null>
      : (...args: Arguments) => Output | null
    : never;

  export function validateFunction<T extends (...args: any[]) => any>(
    func: T,
  ): T extends (...args: infer Arguments) => infer Output
    ? Output extends Promise<infer R>
      ? (...args: Arguments) => Promise<IValidation<R>>
      : (...args: Arguments) => IValidation<Output>
    : never;
}