How to remove union type parsing boilerplate?

DecodeExample.elm

---- BOILER-PLATEY version I'd like to make more succint - I've got 15 simple union types like this and don't want to declare decoder for each of them like this..

type Severity
    = INFO
    | WARN
    | ERROR


decodeSeverity : Decoder Severity
decodeSeverity =
    Decode.string
        |> Decode.andThen
            (\str ->
                case str of
                    "INFO" ->
                        Decode.succeed INFO

                    "WARN" ->
                        Decode.succeed WARN

                    "ERROR" ->
                        Decode.succeed ERROR

                    _ ->
                        Decode.fail <| "Unexpected severity '" ++ str ++ "' - expected one of INFO/WARN/ERROR"
            )
            
---- Attempt at improvement by extracting the common pattern
type Severity
    = INFO
    | WARN
    | ERROR
    
severityDecoder : Decoder Severity
severityDecoder =
    Decode.string |> Decode.andThen (makeUnionDecoder [ INFO, WARN, ERROR ])


makeUnionDecoder : List a -> String -> Decoder a
makeUnionDecoder values strToParse =
    List.filter (\val -> toString val == strToParse) values
        |> List.head
        |> \maybeVal ->
            case maybeVal of
                Nothing ->
                    Decode.fail <| "Unexpected response type: '" ++ strToParse ++ "' - expected one of " ++ (toString values)

                Just val ->
                    Decode.succeed val