Thoth crash repro

thoth-crash-repro.fsx

#r "nuget: Thoth.Json.Net"

open Thoth.Json.Net

type User =
    {
        Name : string
        Age : int
    }

module User =

    let decode : Decoder<User> =
        Decode.object (fun get ->
            {
                Name = get.Required.Field "name" Decode.string
                Age = get.Required.Field "age" Decode.int
            }
        )

type Post =
    {
        Title : string
        Abstract : string
    }

module Post =

    let decode : Decoder<Post> =
        Decode.object (fun get ->
        
            // accessing a value and doing something with it
            let abst = get.Required.Field "abstract" Decode.string
            abst |> Seq.head |> printfn "%A"

            {
                Title = get.Required.Field "title" Decode.string
                Abstract = abst
            }
        )

type Data =
    {
        User : User
        Post : Post option
    }

module Data =

    // Get both structures and decode them with their own decoder accordingly
    let decoder : Decoder<Data> =
        Decode.object (fun get ->
            {
                User = get.Required.Field "user" User.decode
                Post = get.Optional.Field "post" Post.decode
            }
        )

let json = """
{
    "data": {
        "user": {
            "name": "Triss Merigold",
            "age": 42
        },
        "post": null
    }
}
"""

Decode.fromString
    (Decode.field "data" Data.decoder)
    json
    
// Prints:
(*
> Decode.fromString
-     (Decode.field "data" Data.decoder)
-     json;;
System.ArgumentNullException: Value cannot be null. (Parameter 'source')
   at Microsoft.FSharp.Collections.SeqModule.Head[T](IEnumerable`1 source) in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 1647
   at FSI_0007.PostModule.decode@116-12.Invoke(IGetters get) in ..scratchpad.fsx:line 118
   at Thoth.Json.Net.Decode.object[value](FSharpFunc`2 builder, String path, JToken v)
   at Thoth.Json.Net.Decode.decodeMaybeNull[T](String path, FSharpFunc`2 decoder, JToken value)
   at Thoth.Json.Net.Decode.optional[value](String fieldName, FSharpFunc`2 decoder, String path, JToken value)
   at Thoth.Json.Net.Decode.-ctor@806-4.Invoke(String path, JToken value)
   at Thoth.Json.Net.Decode.unwrapWith[T](List`1 errors, String path, FSharpFunc`2 decoder, JToken value)
   at Thoth.Json.Net.Decode.-ctor@804-3.Thoth.Json.Net.Decode.IOptionalGetter.Field[a](String fieldName, FSharpFunc`2 decoder)
   at FSI_0007.DataModule.decoder@136-2.Invoke(IGetters get) in ..scratchpad.fsx:line 137
   at Thoth.Json.Net.Decode.object[value](FSharpFunc`2 builder, String path, JToken v)
   at Thoth.Json.Net.Decode.fromValue[T](String path, FSharpFunc`2 decoder, JToken value)
   at Thoth.Json.Net.Decode.fromString[T](FSharpFunc`2 decoder, String value)
   at <StartupCode$FSI_0008>.$FSI_0008.main@() in ..scratchpad.fsx:line 155
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
*)


// If the value is missing, everything works fine.
let json = """
{
    "data": {
        "user": {
            "name": "Triss Merigold",
            "age": 42
        }
    }
}
"""

(*
> Decode.fromString
-     (Decode.field "data" Data.decoder)
-     json;;
val it: Result<Data,string> = Ok { User = { Name = "Triss Merigold"
                                            Age = 42 }
                                   Post = None }
*)