KeyDown-Event-Decoder

onKeyUp : Json.Decoder Msg
onKeyUp =
    Json.oneOf
        [ logged "keyDecoder" keyDecoder
          --               , charCodeDecoder -- only on keypress
        , logged "keyCodeDecoder" keyCodeDecoder
        , logged "whichDecoder" whichDecoder
        , logged "keyIdentifierDecoder" keyIdentifierDecoder
        , logged "charDecoder" charDecoder
        ]
        `andThen` (\kc -> Json.succeed (OnKey kc))


logged : String -> Json.Decoder a -> Json.Decoder a
logged message decoder =
    decoder `andThen` (\value -> Json.succeed <| Debug.log message value)


keyDecoder : Json.Decoder Int
keyDecoder =
    "key" := Json.string `andThen` succeedIfNot "Unidentified"


keyCodeDecoder : Json.Decoder Int
keyCodeDecoder =
    "keyCode" := Json.int `andThen` validCode


charCodeDecoder : Json.Decoder Int
charCodeDecoder =
    "charCode" := Json.int `andThen` validCode


keyIdentifierDecoder : Json.Decoder Int
keyIdentifierDecoder =
    "keyIdentifier" := Json.int `andThen` validCode


whichDecoder : Json.Decoder Int
whichDecoder =
    "which" := Json.int `andThen` validCode


charDecoder : Json.Decoder Int
charDecoder =
    "char" := Json.string `andThen` succeedIfNot ""


succeedIfNot : String -> String -> Json.Decoder Int
succeedIfNot s dontBeThat =
    if s == dontBeThat then
        Json.fail s
    else
        fromString s


validCode : Int -> Json.Decoder Int
validCode s =
    if s == 0 then
        Json.fail "0"
    else
        Json.succeed s


fromString : String -> Json.Decoder Int
fromString s =
    case String.uncons s of
        Nothing ->
            Json.fail "invalid string"

        Just ( c, s' ) ->
            Json.succeed <| Char.toCode c