Decode Maybe type

Foo.elm

module Foo exposing (..)

import Json.Decode exposing (..)
import Json.Decode.Pipeline exposing (..)


type alias Boat =
    { color : Maybe Color
    }


type Color
    = Red
    | Blue


colorDecoder : String -> Decoder Color
colorDecoder str =
    case str of
        "red" ->
            succeed Red

        "blue" ->
            succeed Blue

        _ ->
            fail (str ++ " color does not exists.")


boatDecoder : Decoder Boat
boatDecoder =
    decode Boat
        |> required "color"
            (nullable
                (string |> andThen colorDecoder)
            )