Elm complex custom JSON decoder -

i have need decode json elm type 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 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" not present , email_id/id/token not present.

how type of conditional decoding in elm

json.decode.andthen lets conditional parsing based on value of field. in case, looks you'll first want pull out value of "status" field, andthen handle separately based on whether 1 or 0.

edit 2016-12-15: updated elm-0.18

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