Elm - decoding a recursive multiway tree

Question

I'm working on a recursive tree of this type

type Node anyType
  = Leaf Id (Maybe anyType) Name
  | Tree Id (List (Node anyType)) Name

where

type Id
  = Id Int
  | Root

and I'm trying to decode a json of this kind into it

{
  "id": "root",
  "entries": [
    {
      "id": 1,
      "value": 0,
      "name": "New Entry"
    },
    {
      "id": 2,
      "entries": [
        {
          "id": 4,
          "value": 0,
          "name": "New Entry"
        }
      ],
      "name": "New Entry"
    }
  ],
  "name": "Budget"
}

To decode the Id type I'm using these decoders

rootDecoder =
(Decode.field "id" Decode.string)
    |> Decode.andThen
        (\str ->
            if str == "root" then
                (Decode.succeed Root)

            else
                Decode.fail <| "[exactMatch] tgt: " ++ "root" ++ " /= " ++ str
        )


intIdDecoder =
    Decode.map Id (Decode.field "id" Decode.int)


idDecoder =
    Decode.oneOf
        [ rootDecoder
        , intIdDecoder
        ]

To decode the tree structure i tried the following, using Json.Decode.Pipeline:

leafDecoder valueDecoder =
    Decode.succeed Leaf
        |> required "id" idDecoder
        |> required "value" valueDecoder
        |> required "name" Decode.string


treeDecoder valueDecoder =
    Decode.succeed Tree
        |> required "id" idDecoder
        |> required "entries"
            (Decode.list
                (Decode.lazy
                    (\_ ->
                        Decode.oneOf
                            [ leafDecoder valueDecoder
                            , treeDecoder valueDecoder
                            ]
                    )
                )
            )
        |> required "name" Decode.string

But when I try to decode the structure I get the following error:

The Json.Decode.oneOf at json.budget.entries[0] failed in the following 2 ways: (1) The Json.Decode.oneOf at json.id failed in the following 2 ways: (1) Problem with the given value: 1 Expecting an OBJECT with a field named `id` (2) Problem with the given value: 1 Expecting an OBJECT with a field named `id` (2) Problem with the given value: { "id": 1, "value": 0, "name": "New Entry" } Expecting an OBJECT with a field named `entries`

But I don't understand why since both the field id and entries are there, and yet it complains.

What am I doing wrong?

Thanks in advance for the help

Answer 1

The problem is that both rootDecoder and intIdDecoder are defined as looking for a field named "id" in an object via Decode.field "id" .... Inside treeDecoder, you are first fetching the "id" field, so your decoder is valid for some JSON like this

// Not what you're looking for
{
  "id": {
    "id": ...
  },
  ...
}

You can fix this by removing the Decode.field "id" portion in those decoders:

rootDecoder = Decode.string
    |> Decode.andThen
        (\str ->
            if str == "root" then
                (Decode.succeed Root)

            else
                Decode.fail <| "[exactMatch] tgt: " ++ "root" ++ " /= " ++ str
        )


intIdDecoder =
    Decode.map Id Decode.int