Testing Elm concurrency with a kind of extreme example.

Test.elm

module Main exposing (main)

import Html exposing (Html, br, div, input, text)
import Html.Attributes exposing (defaultValue, type_, value)
import Html.Events exposing (onInput)

-- Elm 0.18 implementation
-- Example can be found on https://ellie-app.com/sbw95cJgz2a1
-- MODEL


type alias Model =
    { inputValues : List String
    }


init : ( Model, Cmd Msg )
init =
    ( { inputValues = List.repeat 5000 ""
      }
    , Cmd.none
    )


-- UPDATE


type Msg
    = NoOp
    | CaptureInput Int String


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        CaptureInput index val ->
            let
                _ =
                    Debug.log "val" <| String.length val

                updatedModel =
                    { model
                        | inputValues =
                            List.take index model.inputValues
                                ++ [ val ]
                                ++ List.drop (index + 1) model.inputValues
                    }
            in
            ( updatedModel
            , Cmd.none
            )

        NoOp ->
            ( model, Cmd.none )


-- VIEW


textHtml : String -> List (Html Msg)
textHtml val =
    [ br [] []
    , text val
    ]


inputHtml : Int -> String -> List (Html Msg)
inputHtml index val =
    [ br [] []
    , text val
    , br [] []

    {- Transform this to an "uncontrolled" component in order
       to solve the concurrency problem here. ie remove the
       `value` attribute.
    -}
    , input [ type_ "text", onInput (CaptureInput index), value val ] []
    ]


view : Model -> Html Msg
view model =
    div []
        ([ text "Race Condition checking" ]
            ++ (List.concat <| List.indexedMap inputHtml model.inputValues)
            ++ (List.concat <| List.map textHtml model.inputValues)
        )


-- PROGRAM


main : Program Never Model Msg
main =
    Html.program
        { view = view
        , init = init
        , update = update
        , subscriptions = always Sub.none
        }

Test2.elm

module Main exposing (main)

import Browser
import Html exposing (Html, br, div, input, text)
import Html.Attributes exposing (type_, value)
import Html.Events exposing (onInput)

-- Elm 0.19 implementation ( throws a "Maximum CallStack size exceeded" runtime error. Need to take a look )
-- MODEL

type alias Model =
    { inputValues : List String
    }


initialModel : ( Model, Cmd Msg )
initialModel =
    ( { inputValues = List.repeat 5000 ""
      }
    , Cmd.none
    )


-- UPDATE


type Msg
    = NoOp
    | CaptureInput Int String


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        CaptureInput index val ->
            let
                _ =
                    Debug.log "val" <| String.length val

                updatedModel =
                    { model
                        | inputValues =
                            List.take index model.inputValues
                                ++ [ val ]
                                ++ List.drop (index + 1) model.inputValues
                    }
            in
            ( updatedModel
            , Cmd.none
            )

        NoOp ->
            ( model
            , Cmd.none
            )


-- VIEW 


textHtml : String -> List (Html Msg)
textHtml val =
    [ br [] []
    , text val
    ]


inputHtml : Int -> String -> List (Html Msg)
inputHtml index val =
    [ br [] []
    , text val
    , br [] []

    {- Transform this to an "uncontrolled" component in order
       to solve the concurrency problem here.
    -}
    , input [ type_ "text", onInput (CaptureInput index), value val ] []
    ]


view : Model -> Html Msg
view model =
    div []
        ([ text "Race Condition checking" ]
            ++ (List.concat <| List.indexedMap inputHtml model.inputValues)
            ++ (List.concat <| List.map textHtml model.inputValues)
        )


-- PROGRAM


main : Program () Model Msg
main =
    Browser.element
        { init = \_ -> initialModel
        , view = view
        , update = update
        , subscriptions = \_ -> Sub.none
        }