Serializing F# Discriminated union as strings with Json.NET

Question

I'm trying to do a one way transform from F#'s discriminated union to strings upon serialization instead of the default `"Case": [value]". Being able to deserialize the value again is not an issue. Maybe possible with Json.NET?

// Fsharp 4.1.0
open Newtonsoft.Json // 10.0.3

type HowLame =
| PrettyLame
| SuperLame

type Lame = {
    howLame: HowLame;
}

[<EntryPoint>]
let main argv =
    let lame = { howLame = PrettyLame }
    let ser = JsonConvert.SerializeObject(lame)

    // {"soLame":{"Case":"PrettyLame"}} by default
    printfn "%s" ser

    // Desired
    assert (ser = """{"soLame":"PrettyLame"}""")
    0 // return an integer exit code

Answer 1

If you are willing to make the DU an enum (by specifying explicit values, which probably is OK since there is no 'payload'), you can use the standard StringEnumConverter:

#r "../packages/Newtonsoft.Json/lib/net45/Newtonsoft.Json.dll"
open Newtonsoft.Json

type HowLame = PrettyLame=0 | SuperLame=1
type Lame = { howLame: HowLame; }

// in contrast to DUs, enums must be qualified, i.e. Enum.Value
let lame = { howLame = HowLame.PrettyLame }

let settings = JsonSerializerSettings()
settings.Converters.Add(Converters.StringEnumConverter())

let ser = JsonConvert.SerializeObject(lame, settings)
// val ser : string = "{"howLame":"PrettyLame"}"