flow has no Maybe type

flow-maybe.js

// @flow

// https://flow.org/en/docs/types/unions/#toc-disjoint-unions
type Response = {
  success: boolean,
  value?: boolean,
  error?: string
};

// works. 
// but no Maybe type like [Elm](http://package.elm-lang.org/packages/elm-lang/core/latest/Maybe)
// flow will enforce check for nulls. but Elm is much nicer as there is a type :-)
function handleResponse1(response: Response) {
  if (response.success) {
    if (response.value) {
    	var value: boolean = response.value;
    }
  } else {
    if (response.error) {
    	var error: string = response.error;
    }
  }
}

// fails. 
function handleResponse2(response: Response) {
  if (response.success) {
  	var value1: boolean = response.value;
  	var value2: boolean | void = response.value; // works
  } else {
  	var error1: string = response.error;
  	var error2: string | void = response.error;  // works
  }
}

/////////////////////////////////////////////////
// Error: src/main.js:28
//  28:   	var value1: boolean = response.value;
//        	                      ^^^^^^^^^^^^^^ undefined. This type is incompatible with
//  28:   	var value1: boolean = response.value;
//        	            ^^^^^^^ boolean
//
// Error: src/main.js:31
//  31:   	var error1: string = response.error;
//        	                     ^^^^^^^^^^^^^^ undefined. This type is incompatible with
//  31:   	var error1: string = response.error;
//        	            ^^^^^^ string