Nested Tagged Unions

TypeScript Version: 2.5.2

Code

type A = { type: "a", a: number }
type B = { type: "b", b: number }

type X = { type: A, a: string }
type Y = { type: B, b: string }

let x: X | Y

if (x.type.type === "a") {
	x.a // Type Error
}

Expected behavior:

I would expect x to have type X inside the if-statement after disambiguating the nested tagged union types.

Actual behavior:

[pzavolinsky]

👋 currently there is a way (with some heavy type foo) to actually get this to work, I’ll use @rakeshpai example, but others should work as well:

// The nested tagged union type
type X =
  | { type: { name: 'foo' }; settings: { a: boolean; b: string } }
  | { type: { name: 'bar' }; settings: { c: string[] } }
  | { type: { name: 'baz' }; settings: { d: number[] } };

// ========================================================================== //

// The type-level function that, given a name, returns the type(s) within the union for that name.
// The second argument is required to get "distributive conditional types" to work (more on this after
// this code snippet)
type GetTypeForName<TName, TX = X> = TX extends { type: { name: TName } }
  ? TX
  : never;

// Type predicate to run the narrowing in the outer object based on a nested discriminant
const is = <TName extends X['type']['name']>(
  x: X,
  name: TName,
): x is GetTypeForName<TName> => x.type.name === name;

// ========================================================================== //

// The function you actually wanted to write, all of the above is write-once boilerplate
const test = (x: X) => {
  // What you would like out of the box:
  // if (x.type.name === 'baz') console.log(x.settings.d);

  // What you can get, for now:
  if (is(x, 'baz')) console.log(x.settings.d);
};

You can read more about distributive conditional types here.

Also, I covered some of the building blocks for this solution ☝️ in this fan fiction story about typescript metaprogramming.

Hope this helps!

[tejasmanohar]

+1 here. I debugged this for a bit to realize that unions can only be discriminated at the top level / direct. I’m also copying fields down the chain for this purpose