ELM how to decode different value in json array

Question

I have a JSON array with different value in the array and I don't know how to parse it. Here is an example:

[
  {
    "firstname": "John",
    "lastname": "Doe",
    "age": 30
  },
  {
    "companyName": "Doe enterprise",
    "location": "NYC",
    "numberOfEmployee": 10
  }
]

So my JSON is like this, the first element of the array is an user, and the second a company. I have the equivalent in Elm:

type alias User =
  { firsname : String
  , lastname : String
  , age : Int
  }

type alias Company =
  { companyName : String
  , location : String
  , numberOfEmployee : Int
  }

then: Task.perform FetchFail FetchPass (Http.get decodeData url).

So how do I get my User and Company pass in my FetchPass function ? There is something like Json.Decode.at but it's for object only. Here there a way to do something like this ?

decodeData =
  Json.at [0] userDecoder
  Json.at [1] companyDecoder

Answer 1

Json.at works for array indexes as well. First you'll need a Data type to hold the user and company:

import Json.Decode as Json exposing ((:=))

type alias Data =
  { user : User
  , company : Company
  }

And you'll need simple decoders for user and company:

userDecoder : Json.Decoder User
userDecoder =
  Json.object3 User
    ("firstname" := Json.string)
    ("lastname" := Json.string)
    ("age" := Json.int)

companyDecoder : Json.Decoder Company
companyDecoder =
  Json.object3 Company
    ("companyName" := Json.string)
    ("location" := Json.string)
    ("numberOfEmployee" := Json.int)

And finally you can use Json.at to get the values at those array indexes. The difference from your example is that you need to pass a string containing an integer index instead of an int:

dataDecoder : Json.Decoder Data
dataDecoder =
  Json.object2 Data
    (Json.at ["0"] userDecoder)
    (Json.at ["1"] companyDecoder)