example of message passing stage bug

example.elm

-- https://ellie-app.com/9pLFFWd4m7wa1

module Main exposing (..)

import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)
import Process
import Task


main =
  Browser.element
    { init = init
    , view = view
    , update = update
    , subscriptions = subscriptions
    }


type alias Model = 
  { simple : Int
  , passed : Int
  }


init : () -> (Model, Cmd Msg)
init _ =
  ( { simple = 0, passed = 0 }, Cmd.none )


type Msg
  = ModifyState Int
  | FinishState Int


update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    ModifyState i ->
      ( model, Task.perform (\_ -> FinishState i) ((Process.sleep (400))) )
    
    FinishState i ->
      ( { model | simple = model.simple + 1, passed = i }
      , Cmd.none
      )


view : Model -> Html Msg
view model =
  div []
    [ button [ onClick (ModifyState (model.passed + 1))  ] [ text "+" ]
    , div [] [ text (String.fromInt model.simple) ]
    , div [] [ text (String.fromInt model.passed) ]
    ]

subscriptions : Model -> Sub Msg
subscriptions model =
  Sub.none