Debounce with elm in let expressions

I’m tyring to understand why is this not working. I’m trying to debounce, but not a user event from view. By idea this should go into the continiuos flow, which will happen once, but every few seconds. Main idea of this architecture is that events might be triggered from various places, but it will happen only once. I have made a simple example app:

module Main exposing (main)

import Html exposing (Html)


import Html
import Process
import Task
import Debug
import Time
import Control exposing (Control)
import Control.Debounce as Debounce


main : Program Never Model Msg
main =
    Html.program
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
        }


type alias Model =
    { counter : Int
    , state : Control.State Msg
    }


init : ( Model, Cmd Msg )
init =
  { counter = 0, state = Control.initialState }
  ! [ delay (Time.second * 3) <| ContinuousDebouncing ]


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


type Msg
    = Deb (Control Msg)
    | ContinuousDebouncing


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of

        Deb debMsg ->
            Control.update (\s -> { model | state = s }) model.state debMsg

        ContinuousDebouncing ->
            let
                x = Debug.log "ContinuousDebouncing"
                _ = debounce ContinuousDebouncing
            in
                ( { model | counter = model.counter + 1 }, Cmd.none )


debounce : Msg -> Msg
debounce =
    let
        x = Debug.log "debounce"
    in
        Debounce.trailing Deb (3 * Time.second)


delay : Time.Time -> msg -> Cmd msg
delay time msg =
  Process.sleep time
  |> Task.andThen (always <| Task.succeed msg)
  |> Task.perform identity


view : Model -> Html Msg
view model =
    Html.text (toString model.counter)

https://ellie-app.com/tvQ3L6dGrqa1

Solution

In your example app, you only fired the ContinuousDebouncing msg once in the init function, so as expected, the counter only increments once. You probably want to fire ContinuousDebouncing again in the update function.

I think this achieves what you're after:

module Main exposing (main)

import Html exposing (Html)


import Html
import Process
import Task
import Debug
import Time
import Control exposing (Control)
import Control.Debounce as Debounce


main : Program Never Model Msg
main =
    Html.program
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
        }


type alias Model =
    { counter : Int
    , state : Control.State Msg
    }


init : ( Model, Cmd Msg )
init =
  { counter = 0, state = Control.initialState }
  ! [ incrementCounter ]

incrementCounter : Cmd Msg
incrementCounter = Cmd.map debounce <| delay (Time.second * 3) <| ContinuousDebouncing

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


type Msg
    = Deb (Control Msg)
    | ContinuousDebouncing


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of

        Deb debMsg ->
            Control.update (\s -> { model | state = s }) model.state debMsg

        ContinuousDebouncing ->
            let
                x = Debug.log "ContinuousDebouncing"
            in
                ( { model | counter = model.counter + 1 }, incrementCounter )


debounce : Msg -> Msg
debounce =
    let
        x = Debug.log "debounce"
    in
        Debounce.trailing Deb (3 * Time.second)


delay : Time.Time -> msg -> Cmd msg
delay time msg =
  Process.sleep time
  |> Task.andThen (always <| Task.succeed msg)
  |> Task.perform identity


view : Model -> Html Msg
view model =
    Html.text (toString model.counter)

https://ellie-app.com/tPymgfNwYda1