Trying to use disjoint unions in Flow

flow-recursion-limit.js

/* @flow */
// Modeled after https://github.com/facebook/flow/issues/1664#issuecomment-222934455
type AnimalT = {
  id: number,
  name: string
};
type CatT = AnimalT & {
  isPurring: bool
};
type DogT = AnimalT & {
  isBarking: bool
};


// This is cool.
const garfield:AnimalT = {
  id: 42,
  name: 'Garfield',
  isPurring: true
};
console.log(garfield);

// This is cool.
const rex:DogT = {
  id: 45,
  name: 'Rex',
  isBarking: false
};
console.log(rex);

// Trying to check for a property and then work with a more
// specific type doesn't seem to work.
if (garfield.isPurring) {
  const cat:CatT = garfield;
  console.log(cat);
}

// Nor does the approach in https://github.com/facebook/flow/issues/1664#issuecomment-222934455
if (garfield.isPurring) {
  (garfield:CatT);
  console.log(garfield);
}

I also tried adding in an explicit sentinel value (versus matching on the fields that are present). I was trying to match the approach in the docs here: https://flowtype.org/docs/disjoint-unions.html

type Result = Done | Error; // a disjoint union type with two cases
type Done = { status: 'done', answer: Matrix };
type Error = { status: 'error', message: string };

This didn't work either, and my read of the error message is that even expressing the disjoint union was a problem (before even getting to checking the real code)

Code:

type CatT = {
  id: number,
  name: string,
  type: 'cat',
  isPurring: bool
};
type DogT = {
  id: number,
  name: string,
  type: 'dog',
  isBarking: bool
};
type AnimalT = CatT | DogT;


// This is cool (intentionally generic type)
const garfield:AnimalT = {
  id: 42,
  name: 'Garfield',
  type: 'cat',
  isPurring: true
};
console.log(garfield);

// This is cool.
const rex:DogT = {
  id: 45,
  name: 'Rex',
  type: 'dog',
  isBarking: false
};
console.log(rex);

// Trying to check for a property and then work with a more
// specific type doesn't seem to work.
if (garfield.type === 'cat') {
  const cat:CatT = garfield;
  console.log(cat);
}

// Nor does the approach in https://github.com/facebook/flow/issues/1664#issuecomment-222934455
if (garfield.isPurring) {
  (garfield:CatT);
  console.log(garfield);
}

This led to these errors:

demo.js:6
  6:   type: 'cat',
             ^^^^^ string literal `cat`. Expected string literal `dog`, got `cat` instead
 12:   type: 'dog',
             ^^^^^ string literal `dog`

demo.js:12
 12:   type: 'dog',
             ^^^^^ string literal `dog`. Expected string literal `cat`, got `dog` instead
  6:   type: 'cat',
             ^^^^^ string literal `cat`

demo.js:45
 45:   (garfield:CatT);
                 ^^^^ property `isPurring`. Property not found in
 45:   (garfield:CatT);
        ^^^^^^^^ object type

Interestingly, the switch statement works and also detects the type error:

switch (garfield.type) {
  case 'cat': console.log(garfield.isPurring); break;
  case 'dog': console.log(garfield.isBarking); break;
}

and changing them both to isBarking yields:

 51:   case 'cat': console.log(garfield.isBarking); break;
                                        ^^^^^^^^^ property `isBarking`. Property not found in
 51:   case 'cat': console.log(garfield.isBarking); break;
                               ^^^^^^^^ object type