Pretty printing DU recursively

printdu.fs

open System.Text
open Microsoft.FSharp.Reflection

let printDu du =
    
    let rec inner (builder: StringBuilder, duValue: obj, duType: Type) =
        
        if not (FSharpType.IsUnion duType) then failwith "FU!"
              
        let case, values = FSharpValue.GetUnionFields(duValue, duType)
        
        if values.Length > 1 then failwith "FU!"
        let value = values.[0]
        let valueType = value.GetType()
        builder.Append case.Name |> ignore
        
        if FSharpType.IsUnion valueType then
            builder.Append(" -> ") |> ignore
            inner (builder, value, valueType)
        else
            builder.Append (" " + value.ToString()) |> ignore
            builder.ToString()
    inner (StringBuilder(), du, du.GetType())
    
printDu (Some (Ok 1)) //"Some -> Ok 1"