gistfile1.txt

/* @flow */

export function foo(test: boolean, callback: () => Array<number>): Array<number|string> {
  if (test) return [1, '1'];
  return callback();
}

  3: export function foo(test: boolean, callback: () => Array<number>): Array<number|string> {
                                                                                     ^^^^^^ string. This type is incompatible with
  3: export function foo(test: boolean, callback: () => Array<number>): Array<number|string> {
                                                              ^^^^^^ number