[Typescript] StateValue is not typesafe

[mrchantey]

Description

Working with machine.transition() and other implementations of StateValue, the type is not provided. The type safety is provided elsewhere, ie state.matches:

TSV extends TResolvedTypesMeta extends TypegenEnabled
      ? Prop<Prop<TResolvedTypesMeta, 'resolved'>, 'matchesStates'>
      : never

It would be great to see this applied more broadly.

Expected result

Here is an example implementation with a generic StateValue. Naturally a lot of this code is to get at the typegen that would be already available within the StateNode class.

type AnyTypegen = ResolveTypegenMeta<any, any, any, any>;
type GetTypegen<T extends AnyStateMachine> = T extends StateMachine<any,any,any,any,any,any,infer T2>
  ? T2 extends AnyTypegen ? T2["resolved"] : never : never;
type StateValue<T extends AnyStateMachine> = GetTypegen<T>["matchesStates"];

const transitionSafely = <T extends AnyStateMachine>(
  machine: T,
  state: StateValue<T>,
  event: any
) => machine.transition(state, event).value as StateValue<T>;

Actual result

//'foo' should be a typescript error, and value should be 'inactive|active'
const value1 = machine.transition("foo", "TOGGLE").value;

Reproduction

https://codesandbox.io/s/xstate-typescript-demo-le9xqo?file=/src/index.ts

Additional context

No response

[davidkpiano]

This is something we'll be able to do with const type generic parameters in version 5 (cc. @Andarist)

[Andarist]

That's unfortunately not necessarily true. Remember that TS still doesn't support partial inference. This particular issue mentions that it's using typegen and with that, we could revamp this PR at some point: statelyai/xstate-tools#160

[davidkpiano]

This is being addressed in #3940

[Andarist]

We just landed this as a new feature: #4498 . Note that it requires you to use the setup API (that was also recently introduced to v5)