caseStrategy doesn't affect DU case names

[oli]

Using auto coders, the caseStrategy parameter doesn't work for discriminated union case names.

Repro:

#r "nuget:Thoth.Json.Net"
open Thoth.Json.Net
type Direction2 = Up | Down | Left | Right
let inline encoder<'T> = Encode.Auto.generateEncoder<'T>(caseStrategy = CamelCase)
printfn "%s" (encoder Left |> Encode.toString 4)

Expected "left" but result is "Left"

[oli]

The docs say

Decode.Auto helpers accept an optional argument caseStrategy that applies to keys

and DU case names are values in the json not keys, so I guess it's technically not a bug, but it's not what I'd expected.

[Maxime Mangel]

Hello @olivercoad,

as you found caseStrategy control the keys. It only has effect on the record properties.

If your DUs case don't have any argument, you can convert it into a [<StringEnum>] in Fable.

https://fable.io/docs/communicate/js-from-fable.html#StringEnum

The rules applied to the string enum attributes will be respected:

Thoth.Json/tests/Encoders.fs

Lines 611 to 631 in 7f6b448

  #if !NETFRAMEWORK 
  testCase "Encode.Auto.toString works with [<StringEnum>]" <| fun _ -> 
      let expected = "\"firstPerson\"" 
      let actual = Encode.Auto.toString(0, Camera.FirstPerson) 
      equal expected actual 
  
  testCase "Encode.Auto.toString works with [<StringEnum(CaseRules.LowerFirst)>]" <| fun _ -> 
      let expected = "\"react\"" 
      let actual = Encode.Auto.toString(0, Framework.React) 
      equal expected actual 
  
  testCase "Encode.Auto.toString works with [<StringEnum(CaseRules.None)>]" <| fun _ -> 
      let expected = "\"Fsharp\"" 
      let actual = Encode.Auto.toString(0, Language.Fsharp) 
      equal expected actual 
  
  testCase "Encode.Auto.toString works with [<StringEnum>] + [<CompiledName>]" <| fun _ -> 
      let expected = "\"C#\"" 
      let actual = Encode.Auto.toString(0, Language.Csharp) 
      equal expected actual 
  #endif 

If you never used String enum here is the types definitions used in the test above:

Thoth.Json/tests/Types.fs

Lines 252 to 268 in 7f6b448

 #if !NETFRAMEWORK 
 [<StringEnum>] 
 type Camera = 
     | FirstPerson 
     | ArcRotate 
     | IsometricTopDown 
  
 [<StringEnum(CaseRules.LowerFirst)>] 
 type Framework = 
     | React 
     | VueJs 
  
 [<StringEnum(CaseRules.None)>] 
 type Language = 
     | Fsharp 
     | [<CompiledName("C#")>] Csharp 
 #endif 

[oli]

Thanks, that works for my use case.

Closing as if a DU has arguments preventing use of StringEnum, then it probably needs a custom encoder anyway if it needs to conform to an existing API spec.