如何在 Elm 中打印所选选项的索引？

我有一个<select> https://developer.mozilla.org/en-US/docs/Web/HTML/Element/selectHTML 元素有 3 个选项和一个<p> https://developer.mozilla.org/en-US/docs/Web/HTML/Element/p元素。在里面<p>元素我想打印当前所选项目的索引<select>。例如。如果我选择第一个选项，它应该打印 0，如果我选择第二个选项，它应该打印 1，依此类推。如何从下面给出的最小代码开始？

import Html as H exposing (Html)
import Maybe
import Signal as S exposing (Address, (<~))

type alias Model = { selected : Maybe Int }
model = { selected = Nothing }

type Action = NoOp | Select Int
update action model =
  case action of
    NoOp -> model
    Select n -> { model | selected <- Just n }

view address model =
  H.div []
     [ H.select [] [ H.option [] [ H.text "0" ]
                   , H.option [] [ H.text "1" ]
                   , H.option [] [ H.text "2" ]
                   ]
     , H.p [] [ H.text <| Maybe.withDefault ""
                   <| Maybe.map toString model.selected ]
     ]

actions = Signal.mailbox NoOp
main = view actions.address <~ S.foldp update model actions.signal

有很多不同的事件 http://package.elm-lang.org/packages/elm-lang/html/2.0.0/Html-Events in elm-html 2.0.0 http://package.elm-lang.org/packages/elm-lang/html/2.0.0，但与<select>HTML 元素。所以你肯定需要一个自定义事件处理程序，你可以使用它来创建on http://package.elm-lang.org/packages/elm-lang/html/2.0.0/Html-Events#on。它有一个类型：

on : String -> Decoder a -> (a -> Message a) -> Attribute

每次选择里面的选项时都会触发的事件<select>叫做“change” https://developer.mozilla.org/en-US/docs/Web/Events/change。你需要的是目标选定索引 http://package.elm-lang.org/packages/elm-community/html-extra/2.2.0/Html-Events-Extra#targetSelectedIndex from 榆树社区/html-extra http://package.elm-lang.org/packages/elm-community/html-extra它利用了一个selectedIndex https://docs.webplatform.org/wiki/dom/HTMLElement/selectedIndex财产。

最终代码如下所示：

更新至 Elm-0.18

import Html exposing (..)
import Html.Events exposing (on, onClick)
import Html.Attributes exposing (..)
import Json.Decode as Json
import Html.Events.Extra exposing (targetSelectedIndex)


type alias Model =
    { selected : Maybe Int }


model : Model
model =
    { selected = Nothing }


type Msg
    = NoOp
    | Select (Maybe Int)


update : Msg -> Model -> Model
update msg model =
    case msg of
        NoOp ->
            model

        Select s ->
            { model | selected = s }


view : Model -> Html Msg
view model =
    let
        selectEvent =
            on "change"
                (Json.map Select targetSelectedIndex)
    in
        div []
            [ select [ size 3, selectEvent ]
                [ option [] [ text "1" ]
                , option [] [ text "2" ]
                , option [] [ text "3" ]
                ]
        , p []
            [ text <|
                Maybe.withDefault "" <|
                    Maybe.map toString model.selected
            ]
        ]


main : Program Never Model Msg
main =
    beginnerProgram { model = model, view = view, update = update }

您可以在此处的浏览器中运行它https://runelm.io/c/xum https://runelm.io/c/xum