todo.elm

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

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

type alias Todo =
  { id: Int
  , isDone: Bool
  , task: String
  }

type alias Model = List Todo
init : Model
init =
  [ { id = 1, isDone = False, task = "My first todo" }
  , { id = 2, isDone = False, task = "My second todo" }
  , { id = 3, isDone = False, task = "My third todo" }
  ]

type Msg = MarkDone Int

update : Msg -> Model -> Model
update msg model =
  case msg of
    MarkDone id ->
      List.map
        (ifElse (\a -> a.id == id) (\a -> { a | isDone = not a.isDone }) identity)
        model

view : Model -> Html Msg
view model =
  div [] (List.map createTodo model)

createTodo : Todo -> Html Msg
createTodo todo =
  div
    [ onClick (MarkDone todo.id) ]
    [ text ((ifElse .isDone (always "[x] ") (always " [ ] ") todo) ++ todo.task) ]

ifElse : (a -> Bool) -> (a -> b) -> (a -> b) -> a -> b
ifElse conditionFn trueFn falseFn value =
  if (conditionFn value) then
    trueFn value
  else
    falseFn value