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Exhaustive checking with empty type

flow.org/try

type Cases = 'A' | 'B';

function exhaust(x: Cases) {
  if (x === 'A') {
    // do stuff
  } else if (x === 'B') {
    // do different stuff
  } else {
    // only true if we handled all cases
    (x: empty);
  }
}

What happens when you add new case 'C'? You will get this error:

Cannot cast x to empty because string literal C [1] is incompatible with empty [2].

 [1]  5│ function exhaust(x: Cases) {
      6│   if (x === 'A') {
      7│     // do stuff
      8│   } else if (x === 'B') {
      9│     // do different stuff
     10│   } else {
     11│     // only true if we handled all cases
 [2] 12│     (x: empty);
     13│   }
     14│ }
     15│

You can be sure that you covered all the cases this way. Another real-life example:

export function exhaustB(reason: 'magicLink' | 'signUpConfirmation' | 'resetPassword') {
  switch (reason) {
    case 'magicLink':
      return __('account.check_email_magic_link');
    case 'signUpConfirmation':
      return __('account.check_email_sign_up');
    case 'resetPassword':
      return __('account.you_will_recieve_password');
    default:
      return invariant(false, 'Unsupported reason: %j', (reason: empty));
  }
}

Notice how is the empty type used at the same time with reason.