Flow - Exhaustive Switch

flow-exhaustive-switch.js

// from: http://ouicar.github.io/2016/08/08/exhaustive-switch.html
// modified to use Flow comments

/* :: 
type Empty = 'empty type' & 'nothing there'; 
type t = 'a' | 'b';
*/

function unexpectedCase(impossible /* : Empty */) /* : void */ {
  reportError(`Unexpected case ${impossible}`);
}

// This function will type-check if and only if we handle all the cases.
function toNumber(value /* : t */) /* : number */ {
  switch (value) {
    case 'a':
      return 0;
    case 'b':
      return 1;
    default:
      unexpectedCase(value);
      return 0;
  }
}

flow-exhaustive-switch.md

Flow Exhaustive Switch

From: http://ouicar.github.io/2016/08/08/exhaustive-switch.html

demo