Conditional JSON decoding based on a field value

Question

I have a need to decode JSON into an elm type like below:

Type

type User = Anonymous | LoggedIn String

type alias Model =
  { email_id : User
  , id : Id
  , status : Int
  , message : String
  , accessToken : AccessToken
  }

JSON Message 1

{
  "status": 0,
  "message": "Error message explaining what happened in server"
}

into type value

Model {
   "email_id": Anonymous
   , id: 0
   , status: 0
   , message: json.message
   , accessToken: ""
}

JSON Message 2

{
  "status": 1,
  "email_id": "asdfa@asdfa.com"
  "token": "asdfaz.adfasggwegwegwe.g4514514ferf"
  "id": 234
}

into type value

Model {
   "email_id": LoggedIn json.email_id
   , id: json.id
   , status: json.status
   , message: ""
   , accessToken: json.token
}

Decoder information

Above, "message" is not always present and email_id/id/token are always not present.

How to do this type of conditional decoding in elm

Answer 1

Json.Decode.andThen lets you do conditional parsing based on the value of a field. In this case, it looks like you'll first want to pull out the value of the "status" field, andThen handle it separately based on whether it is a 1 or 0.

Edit 2016-12-15: Updated to elm-0.18

import Html as H
import Json.Decode exposing (..)

type User = Anonymous | LoggedIn String

type alias Id = Int

type alias AccessToken = String

type alias Model =
  { email_id : User
  , id : Id
  , status : Int
  , message : String
  , accessToken : AccessToken
  }

modelDecoder : Decoder Model
modelDecoder =
  (field "status" int) |> andThen modelDecoderByStatus

modelDecoderByStatus : Int -> Decoder Model
modelDecoderByStatus status =
  case status of
    0 ->
      map5
        Model
        (succeed Anonymous)
        (succeed 0)
        (succeed status)
        (field "message" string)
        (succeed "")
    1 ->
      map5
        Model
        (map LoggedIn (field "email_id" string))
        (field "id" int)
        (succeed status)
        (succeed "")
        (field "token" string)
    _ ->
      fail <| "Unknown status: " ++ (toString status)

main = H.div []
  [ H.div [] [ decodeString modelDecoder msg1 |> Result.toMaybe |> Maybe.withDefault emptyModel |> toString |> H.text ]
  , H.div [] [ decodeString modelDecoder msg2 |> Result.toMaybe |> Maybe.withDefault emptyModel |> toString |> H.text ]
  ]

emptyModel = Model Anonymous 0 0 "" ""

msg1 = """
{
  "status": 0,
  "message": "Error message explaining what happened in server"
}
"""

msg2 = """
{
  "status": 1,
  "email_id": "asdfa@asdfa.com"
  "token": "asdfaz.adfasggwegwegwe.g4514514ferf"
  "id": 234
}
"""