caseStrategy doesn't affect DU case names

[olivercoad]

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"

[olivercoad]

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.

[MangelMaxime]

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

[olivercoad]

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.