Decoder a -> a?

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You can use something like this : :slight_smile:

decodeMsg jsValue =
    D.field "id" D.string
        |> D.andThen decodePayload

decodePayload id=
    case id of
        "IntPayload" ->
            ... the entire msg to be decoded here, and id is already deoded and available...

        "StringPayload" ->
            D.field "payload" (D.map ItHasStringPayload D.string)

        _ ->
            D.fail <| "Event with unknown id: " ++ id

Edit: I guess for your usecase:

decodeMsg jsValue =
    D.field "id" D.int
        |> D.andThen (\idAsInt ->
            ... your decoder... (id is available as Int here) 
            )

But I would not have any logic in my decoders,
I would have just decoded it as is and then check if the Id match in my update function.

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