Breaking change in v2 related to non-F# objects?

[matthid]

Description

I have not quite debugged our issue but I feel like the upgrade to fable 2 broke the debugger for us

Repro code

This is a bit hard because basically what we do is embed TypeScript "elmish" components (basically update/view/init written in typescript) into our fable emlish application. This worked fine.

The problem I see is this:

The parameter seems to originate from the following Typescript definition:

type Msg =
    { type: "LanguageMapping", translations: object } |
    { type: "GetInstanceName", name: string | undefined } |
    { type: "TestConnectionAvailable" } |
    { type: "ConnectionAvailable", available: boolean } |
    ... several other cases left out ...;

Expected and actual results

Expected: Debugger can work with non-F# objects
Actual: Debugger fails

Related information

• elmish version: Fable.Elmish (2.0.3)
• fable-compiler version: "fable-compiler": { "version": "2.1.10",
• fable-core version: Fable.Core (2.0.3)
• Operating system: Win 7 (I know but I'm pretty sure it is not related to the OS)

[MangelMaxime]

The debugger is now using Thoth.Json as Fable 2 removed toJson and ofJson.

Because the debugger is also using version 2 of Thoth.Json if it doesn't understand the type it will generate an error.

Thoth.Json isn't able to serialize/deserialize an unknown type. However, in version 3 of Thoth.Json, we added the possibility to extends the known types by Thoth.Json docs.

I don't know if this is possible to extend it in order to support non F# types. Perhaps, by defining a binding for the type it can be possible or perhaps I need to allow adding non F# type support to Thoth.Json with the limitation that the type is identifiable using typeof or a test function. In the case of your type, the test function would check for type property for example.

I didn't release Thoth.Json v3 yet because I didn't get a lot of feedback on it. And wanted to finish the docs upgrade.

As a quick fix, I think you will need to copy the debugger function and use Thoth.Json v3 to check if you can deserialize TypeScript types.

[matthid]

Hm, maybe then Thoth is just no applicable for us and we need a way to opt-out and use normal Js-object -> Json serialization?

I can try to find a workaround where I use Thoth for fable components and use "simple" js serialization for typescript components

[et1975]

If you can think of a sensible extensibility point for the debugger to take the serializer that would be great too.

[matthid]

@MangelMaxime apparently this wasn't an issue with Thoth after all. There was just code added which assumed that the type is a F#-DU which it obviously isn't in my scenario...

[MangelMaxime]

Good catch :)