Code for https://ellie-app.com/7DjWTc7DgRGa1

Main.elm

module Main exposing (main)

import Browser
import Html exposing (..)
import Html.Attributes exposing (..)
import Html.Events exposing (..)

init = { a = "1", b = "2" }

type Msg = ChangeA String | ChangeB String

update msg model =
    case msg of
        ChangeA value -> { model | a = value }
        ChangeB value -> { model | b = value }

view model =
    div []
        [ input [ onInput ChangeA, value model.a, type_ "number"] []
        , input [ onInput ChangeB, value model.b, type_ "number"] []
        , p []
            [ text <| String.join " "
                [ model.a
                , "+"
                , model.b
                , "="
                , case Maybe.map2 (+) (String.toFloat model.a) (String.toFloat model.b) of
                    Just value -> String.fromFloat value
                    Nothing -> "undefined"
                ]
            ]
        ]

main = Browser.sandbox { init = init, view = view, update = update }