Elm decoding polymorphic json list

json

    [
      {
        "type": "summary",
        "description": "an awesome summary",
        "title": "an awesome title"
      },
      {
        "type": "video",
        "mpg": "http://vids.com/awesome.mpg"
      }
    ]

model and decoder

module Models.Attachment exposing (Attachment (..), )

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


type alias VideoRecord =
    { mpg : String
    }


type alias SummaryRecord =
    { title : String
    , description : String
    }


type Attachment
    = Video VideoRecord
    | Summary SummaryRecord


videoDecoder =
    JD.object1 VideoRecord
        ("mpg" := JD.string)
        |> JD.map Video


summaryDecoder =
    JD.object2 SummaryRecord
        ("title" := JD.string)
        ("description" := JD.string)
        |> JD.map Summary


decoder =
    JD.oneOf ([ videoDecoder, summaryDecoder ])


listDecoder =
    JD.list decoder

tests

module Models.AttachmentTest exposing (..)

import Test exposing (..)
import Expect
import Json.Decode exposing (decodeString)
import Models.Attachment as Attachment
import Models.Attachment exposing (..)


videoJson =
    """
    {
      "type": "video",
      "mpg": "http://vids.com/awesome.mpg"
    }
    """


summaryJson =
    """
    {
      "type": "summary",
      "description": "an awesome summary",
      "title": "an awesome title"
    }
    """


attachmentsJson =
    """
    [
      {
        "type": "summary",
        "description": "an awesome summary",
        "title": "an awesome title"
      },
      {
        "type": "video",
        "mpg": "http://vids.com/awesome.mpg"
      }
    ]
    """


all : Test
all =
    describe "Attachment"
        [ test "decode video" <|
            \() ->
                Expect.equal
                    (decodeString Attachment.decoder videoJson)
                    (Ok
                        (Video { mpg = "http://vids.com/awesome.mpg" })
                    )
        , test "decode summary" <|
            \() ->
                Expect.equal
                    (decodeString Attachment.decoder summaryJson)
                    (Ok
                        (Summary { description = "an awesome summary", title = "an awesome title" })
                    )
        , test "decode list containing a summary and video" <|
            \() ->
                Expect.equal
                    (decodeString Attachment.listDecoder attachmentsJson)
                    (Ok
                        [ Summary { description = "an awesome summary", title = "an awesome title" }
                        , Video { mpg = "http://vids.com/awesome.mpg" }
                        ]
                    )
        ]