ELM : Passing initial data to ELM application

05 Nov 2017

When we start introducing the ELM slowly into our existing application, there is high chance that we need to pass some initial data to ELM module to configure it while it starts up.

Consider we have written a small application using program.

-- Main.elm
type Action = NoOp

type alias Model =
  { name: String }

initModel : Model
initModel =
  { name = "A" }

view : Model -> Html Action
  div [] [
    text model.name
  ]

update : Action -> Model -> (Model -> Cmd Action)
update action model ->
  case action of
    NoOp -> 
      (model, Cmd.none)

init : (Model -> Cmd Action)
init =
  (initModel, Cmd.none)

main : Program Never Model Action
main =
  Html.program {
    init: init,
    update: update,
    view: view,
    subscription: \_ -> Sub.none
  }

And the bootup will be

import { Main } from './Main.elm'
var app = Main.embed(document.getElementById('elm-app'));

In the above app we don’t have any way to pass the initial value for the name. To make the above app to accept name while booting up we need to follow 2 steps

  1. Pass the data as second parameter to embed
  2. Use programWithFlags instead of program.

Let start with defining the type flags

type alias Flags =
  { name : String }

Now use the programWithFlags instead of program for our main.

main : Program Flags Model Action
main =
  Html.programWithFlags {
    init: init,
    update: update,
    view: view,
    subscription: \_ -> Sub.none
  }

Then, in init method update the model using the value we received via flag.

init : Flags -> (Model -> Cmd Action)
init flags =
  ({ initModel | name = flags.name }, Cmd.none)

Now our app is ready to accept the name on startup. Let’s pass the value to our elm program.

import { Main } from './Main.elm'
var app = Main.embed(document.getElementById('elm-app'), { 
  name: 'ELM' 
});

Thats it we are now passing the data to ELM program while it starting. If we are using fullscreen method, same as embed it will accept the initial data as the second parameter.

So now our final code will look like,

-- Main.elm
type alias Flags =
  { name : String }

type Action = NoOp

type alias Model =
  { name: String }

initModel : Model
initModel =
  { name = "A" }

view : Model -> Html Action
view model =
  div [] [
    text model.name
  ]

update : Action -> Model -> (Model -> Cmd Action)
update action model ->
  case action of
    NoOp -> 
      (model, Cmd.none)

init : Flags -> (Model -> Cmd Action)
init flags =
  ({ initModel | name = flags.name }, Cmd.none)

main : Program Flags Model Action
main =
  Html.programWithFlags {
    init: init,
    update: update,
    view: view,
    subscription: \_ -> Sub.none
  }

Hope it helped.

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