constraints.js

// paste me into https://codemix.github.io/flow-runtime/#/try
// (no gist support yet :( )

import t, {reify} from 'flow-runtime';

type ValidURL = string;

(reify: ValidURL).addConstraint(url => {
  if (/^(.|\/)/.test(url)) {
     return "Don't use relative URLs for some reason"; 
  }
})

function visitURL(url: ValidURL) {
  return url; 
}

console.log(visitURL('./nope'));