Cannot compose late models

[Ghirigoro]

Bug report Using late references in compose fails at runtime. My understanding is that late types can be used as aliases for the type they refer to, but that doesn't seem to be the case here.

NOTE: This may not be a bug but I cannot find any reference to this limitation in the docs

• [X] I've checked documentation and searched for existing issues
• [X] I've made sure my project is based on the latest MST version
• [X] Fork this code sandbox or another minimal reproduction.

Sandbox link or minimal reproduction code

const Base = types.model({ x: types.number });
const Derived = types.compose(
  types.late(() => Base),
  types.model({ y: types.number })
);

// ERROR: [mobx-state-tree] expected mobx-state-tree model type as argument 1, got [object Object] instead 
const test = Derived.create({ x: 1, y: 2 }); 

Describe the expected behavior It should create an instance of the Derived model without errors.

Describe the observed behavior It fails in creating an instance of the Derived model with the error: [mobx-state-tree] expected mobx-state-tree model type as argument 1, got [object Object] instead 

[patrickschmelter]

have you found a solution?

[Ghirigoro]

@patrickschmelter Sorry I haven't and given the level of response this is getting I'm guessing there isn't one ;)

[diegochavez]

Hi @Ghirigoro @patrickschmelter I'm trying to understand your use case. The documentation explains that types.late is intended to do recursive implementations,

Regarding compose you can only use it as adding types.model https://mobx-state-tree.js.org/API/#compose

Check this example I did of the use for types.late and compose https://codesandbox.io/s/elastic-moser-oj5mum?file=/src/store.tsx:121-126

[Ghirigoro]

@diegochavez my memory is a bit hazy on the specifics (I've since moved away from the approach that required this) but if I remember correctly I was writing a DSL to define content for our app which would be used to generate JSON schemas and MST models. The DSL was pretty open ended so you could find yourself with complicated types that were composed of types that had recursive (or transitively recursive) references. Rather than try to unsnarl those and try to figure out which types required late references in composition I made ALL references in composition late. And that's where I ran into the problem.

[Ghirigoro]

No sorry

On Aug 19, 2022 at 5:51 AM, <Patrick Schmelter @.***)> wrote:

have you found a solution?

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[mikeKlech]

I think you should wrap to types.late the whole model. Like this:

const Derived = types.late(() => types.compose(
  Base,
  types.model({ y: types.number })
));

[coolsoftwaretyler]

Hey @Ghirigoro - did @mikeKlech's solution here work for you? Did you find any other solution to the problem?

[Ghirigoro]

@coolsoftwaretyler I had long abandoned that approach by the time that @mikeKlech made his suggestion, so I haven't tried it and I didn't find any other solutions.

[coolsoftwaretyler]

Thanks for the follow up!

For folks who end up here with similar problems, I put together a quick CodeSandbox comparing the original snippet in the issue, and the solution proposed by @mikeKlech. Wrapping everything in types.late does seem to remove the errors, although it's unclear to me if that would have solved the original issue.

I'm going to close this out for now, but please open another issue if anyone has follow up problems.