sample.js

import {sampleOne} from 'babel-plugin-transform-flow-to-gen/api';
import type {Developer, JavaScript} from './types';

const javaScriptDevGen = Developer(JavaScript());

const dev = sampleOne(javaScriptDevGen);

console.log(dev);
// { name: 't', age: 4, preferredLanguage: { name: 'javascript' } }

types.js

export type Developer<T> = {
  name: string,
  age: number,
  preferredLanguage: T
};

export type JavaScript = {
  name: 'javascript'
};

export type Ruby = {
  name: 'ruby'
};