Decoding an Elm type from a json object

DecodeElmType.elm

module DecodeElmType exposing (Status(..), json, statusDecoder, statusFieldToStringDecoder, statusFromStringDecoder)

import Json.Decode exposing (..)


type Status
    = Queued
    | Started
    | Complete


json =
    """ { "status": "Started" } """


statusFieldToStringDecoder : Decoder String
statusFieldToStringDecoder =
    field "status" string


statusFromStringDecoder : Result Error String -> Maybe Status
statusFromStringDecoder result =
    case result of
        Err _ ->
            Nothing

        Ok value ->
            case value of
                "Queued" ->
                    Just Queued

                "Started" ->
                    Just Started

                "Complete" ->
                    Just Complete

                _ ->
                    Nothing


statusDecoder : String -> Maybe Status
statusDecoder str =
    decodeString statusFieldToStringDecoder str |> statusFromStringDecoder



-- > import DecodeElmType exposing (..)
-- > statusDecoder json
-- Just Started : Maybe Status
-- > statusDecoder """ { "status": "Empty" } """
-- Nothing : Maybe Status