elm-history

This library provides a data structure for managing the "History" of some values. It is useful for making features that are along the lines of undo and redo.

import History exposing (History)
import Browser exposing (sandbox)
import Html exposing (text)

type alias Model = History Int
type alias Msg = Increment
               | Decrement
               | Redo
               | Undo

init : Model
init =
    History.new 0

update : Msg -> Model -> Html Msg
update msg model =
    case msg of
        Increment ->
            History.to ((History.current model) + 1) model
        Decrement ->
            History.to ((History.current model) - 1) model
        Undo ->
            History.back model
        Redo ->
            History.forward model
     
view = text (String.fromInt <| History.current model)

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