Elm:如何将Html FooMsg转换为Html Msg

import Html exposing (Html)
import Html.Events exposing (onClick)


-- 模型
type alias Model =
    { count : Int
    }


-- 消息类型
type Msg
    = Increment
    | Decrement


-- 更新函数
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Increment ->
            ( { model | count = model.count + 1 }, Cmd.none )

        Decrement ->
            ( { model | count = model.count - 1 }, Cmd.none )


-- 视图函数
view : Model -> Html Msg
view model =
    Html.div []
        [ Html.button [ onClick Increment ] [ Html.text "+" ]
        , Html.text (String.fromInt model.count)
        , Html.button [ onClick Decrement ] [ Html.text "-" ]
        ]


-- 将Html FooMsg转换为Html Msg
convertHtml : Html FooMsg -> Html Msg
convertHtml html =
    Html.map fooMsgToMsg html


-- 将FooMsg转换为Msg的函数
fooMsgToMsg : FooMsg -> Msg
fooMsgToMsg fooMsg =
    case fooMsg of
        FooIncrement ->
            Increment

        FooDecrement ->
            Decrement