Option fields fail to serialize

[reinux]

Since Fable 5, any return value that includes an Option seems to fail. I think there's disagreement as to how Option should be serialized between the server and client implementations.

In this repro, it produces a different error depending on whether the value is Some or None; in my other project, it fails silently.

When it's None:

Response:

{"thing":null}

Exception {message: 'Cannot convert null to ["Union",null]'}

When it's Some:

Response:

{"thing":"Something"}

Exception {message: "Case Something was not valid for type 'FSharpOptio…', expected one of the cases [ 'None' , 'Some' ]"}

Client.fs:

open Fable.Core
open Fable.Remoting.Client
open Common

Browser.Dom.console.log("Testing")

let client =
  Remoting.createApi()
  |> Remoting.buildProxy<TestApi>

let p =
  promise {
    Browser.Dom.console.log("Starting")
    let! thing = client.getThing() |> Async.StartAsPromise
    Browser.Dom.console.log("Thing", thing)
    return thing
  }

Packages:

    <PackageReference Include="Fable.Promise" Version="3.2.0" />
    <PackageReference Include="Fable.Remoting.Client" Version="8.0.0" />

Server.fs:

open Microsoft.AspNetCore.Builder
open Fable.Remoting.AspNetCore
open Fable.Remoting.Server
open Common

let api =
  Remoting.createApi()
  |> Remoting.fromValue { getThing = fun () -> async { return { thing = None } } }

[<EntryPoint>]
let main (args: string array) =
  let builder = WebApplication.CreateBuilder args
  let app = builder.Build()

  app.UseRemoting api

  app.Run()
  0

Packages:

    <PackageReference Include="Fable.Remoting.AspNetCore" Version="3.1.0" />

Common.fs:

namespace Common

type TestThing =
    { thing: string option }

type TestApi =
    { getThing: unit -> Async<TestThing> }

[reinux]

Remoting.withBinarySerialization turns out to be a viable workaround for now.

[kerams]

Most likely fable-compiler/Fable#4529. Guessing SimpleJson or JSON serialization first checks for unions, then options. Historically Fable didn't consider options unions, so the order didn't matter.

Yep, https://github.com/Zaid-Ajaj/Fable.SimpleJson/blob/0739c372c01834c3ec482100331c56184809b167/src/TypeInfo.Converter.fs#L168-L183

Easily fixed with a PR (although something else might then surface on the Remoting side)

[reinux]

Interesting, I should check all my code too... I think I make some of the same assumptions.

[reinux]

So even with Remoting.withBinarySerialization, any Option in the request payload also bombs out. I'll make a PR.

[mattgallagher92]

Note: a workaround is to use Fable 5.0.0, since fable-compiler/Fable#4529 wasn't included until Fable 5.1.0. Obviously that means missing out on some improvements and fixes (check the linked release notes), but it might be a viable means of unblocking Fable 5 usage with Fable.Remoting until Zaid-Ajaj/Fable.SimpleJson#111 is merged and released.