Thoth.Json: allow decoding of classes when value is null

[0x53A]

I am using Fable Elmish React with ServerSideRendering.

My Model contains a mutable System.Collections.Generic.Dictionary as a cache.

On deserialization of the model I get an exception:

Error: Error at: `$.PageModel.SlabCache` I run into a `fail` decoder: Class types cannot be automatically deserialized: System.Collections.Generic.Dictionary`2[ebosYC.Domain.SlabId,Fable.Import.JS.Promise`1[System.Tuple`2[]]]

When I render the page on the server, I set the cache to null.

I haven't looked at the code of Thoth yet, but what would you think about emiting a special decoder that only accepts null when encountering an unknown class type?

[0x53A]

So I think this would be solved by the new extra/custom serializers in Thoth.Json 3, but I couldn't yet update because of the breaking changes:

1. Elmish.Bridge: Not compatible with Thoth.Json 3-beta Nhowka/Elmish.Bridge#17
2. Powerpack / BrowserStorage (yes I know powerpack was split):

ERROR in ./.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs
Module Error (from ./node_modules/fable-loader/index.js):
c:/Projekte/ebos/ebosYC/.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs(18,40): (18,69) error FABLE: Cannot get type info of generic parameter T, please inline or inject a type resolver
 @ ./src/Client/Client.fs 21:0-302 173:16-54 188:4-42 194:4-42 201:52-92
 @ ./src/Client/App.fs
 @ ./src/Client/Client.fsproj
 @ multi ./src/Client/Client.fsproj ./src/Client/site.css

ERROR in ./.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs
Module Error (from ./node_modules/fable-loader/index.js):
c:/Projekte/ebos/ebosYC/.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs(22,28): (22,35) error FSHARP: The type 'Decoder' is not defined in 'Thoth.Json.Decode'. Maybe you want one of the following:
   DecoderException
 @ ./src/Client/Client.fs 21:0-302 173:16-54 188:4-42 194:4-42 201:52-92
 @ ./src/Client/App.fs
 @ ./src/Client/Client.fsproj
 @ multi ./src/Client/Client.fsproj ./src/Client/site.css

ERROR in ./.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs
Module Error (from ./node_modules/fable-loader/index.js):
c:/Projekte/ebos/ebosYC/.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs(32,42): (32,71) error FABLE: Cannot get type info of generic parameter T, please inline or inject a type resolver
 @ ./src/Client/Client.fs 21:0-302 173:16-54 188:4-42 194:4-42 201:52-92
 @ ./src/Client/App.fs
 @ ./src/Client/Client.fsproj
 @ multi ./src/Client/Client.fsproj ./src/Client/site.css

ERROR in ./.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs
Module Error (from ./node_modules/fable-loader/index.js):
c:/Projekte/ebos/ebosYC/.fable/Fable.PowerPack.3.0.0/BrowserStorage.fs(8,28): (8,35) error FSHARP: The type 'Decoder' is not defined in 'Thoth.Json.Decode'. Maybe you want one of the following:
   DecoderException
 @ ./src/Client/Client.fs 21:0-302 173:16-54 188:4-42 194:4-42 201:52-92
 @ ./src/Client/App.fs
 @ ./src/Client/Client.fsproj
 @ multi ./src/Client/Client.fsproj ./src/Client/site.css

[0x53A]

I changed it to Dictionary<_,_> option and now it works (with 2.x).

As long as the value sent from the server is None, it correctly deserializes.

[MangelMaxime]

Indeed, when using 'T option if you send null the auto decoder will work. It was added for people using F# on client and server side.