How to print index of selected option in Elm? - html-select

Question

I have a <select> HTML element with 3 options and a <p> element. In the <p> element I want to print index of the currently selected item in <select>. E.g. if I select the first option, it should print 0, if I select the second option, it should print 1, and so on. How do I proceed from the minimal code, which is given below?
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

Answer 1

There's a lot of different events in elm-html 2.0.0, but nothing relevant to the <select> HTML element. So you definitely need a custom event handler, which you can create using on. It has a type:
on : String -> Decoder a -> (a -> Message a) -> Attribute
The event that is triggered every time you select an option inside the <select> is called “change”. What you need is targetSelectedIndex from elm-community/html-extra which ustilizes a selectedIndex property.
The final code would look like this:
Updated to 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 }
You can run it in browser here https://runelm.io/c/xum