A simple undo example

Simple Undo Example

import Html exposing (Html, div, text, input, button)
import Html.Attributes exposing (value, placeholder)
import Html.Events exposing (on, targetValue, onClick)
import Signal as Signal
import StartApp
import List exposing (head, tail)

type alias Model = { text:String }

type Action = UpdateText String


update : Action -> Model -> Model
update action model =
  case action of
    UpdateText text -> { model | text <- text }

model = { text = "" }

view : Context -> Model -> Html
view context model =
  div [] [
    input [ on "input" targetValue (Signal.message context.actions << UpdateText ), value model.text, placeholder "Type Something Here!" ] [ ],
    div [] [text model.text],
    button [ onClick context.undo ()] [text "click" ]
  ]
  
  

type HistoryAction = Modify Model Action | Undo ()

type alias HistoryModel = { current:Model, history:List Model }
  
state = { current = model, history = [] }

historyUpdate : HistoryAction -> HistoryModel -> HistoryModel
historyUpdate action model =
  case action of
    Modify current action -> { model | current <- update action model.current, 
                                     history <- model.current :: model.history }
                                     
    Undo action -> { model | current <- case head model.history of 
                                  Just xs -> xs 
                                  Nothing -> model.current, 
                      history <- case tail model.history of
                                  Just xs -> xs
                                  Nothing -> []}
                                  
historyView : Signal.Address HistoryAction -> HistoryModel -> Html
historyView address model =
  view ({
    actions = (Signal.forwardTo address (Modify model.current)), 
    undo = (Signal.forwardTo address Undo)
  }) model.current
  
  
type alias Context = { 
  actions : Signal.Address Action, 
  undo : Signal.Address ()
}


main =
  StartApp.start { model = state, view = historyView, update = historyUpdate }