Dance, Computer, Dance

by Ray Grasso

Structuring a Large Elm Application

21 October, 2016

I’m building an application in Elm and have been working on a strategy for breaking it down into smaller pieces.

My preferred approach is a few minor tweaks to the pattern used in this modular version of the Elm TodoMVC application1.

The Structure

The file structure is as follows.

$ tree src
src
├── Global
│   ├── Model.elm
│   ├── Msg.elm
│   └── Update.elm
├── Main.elm
├── Model.elm
├── Msg.elm
├── TransactionList
│   ├── Components
│   │   ├── FilterForm.elm
│   │   └── TransactionTable.elm
│   ├── Model.elm
│   ├── Msg.elm
│   ├── Update.elm
│   └── View.elm
├── Update.elm
└── View.elm

Global contains global state and messages, and TransactionList is a page in the application.

The top level Model, Msg, Update, and View modules stitch together the lower level components into functions that are passed into the top level Elm application (as shown below).

--
-- Main.elm
--
import Html.App as Html
import Model
import Update
import View

main : Program Never
main =
    Html.program
        { init = Model.init
        , update = Update.updateWithCmd
        , subscriptions = Update.subscriptions
        , view = View.rootView }

--
-- Model.elm
--
module Model exposing (..)

import Global.Model as Global
import TransactionList.Model as TransactionList

type alias Model =
    { global : Global.Model
    , transactionList : TransactionList.Model
    }

init : ( Model, Cmd msg )
init =
    ( initialModel, Cmd.none )

initialModel : Model
initialModel =
    { global = Global.initialModel
    , transactionList = TransactionList.initialModel
    }
  
--
-- Msg.elm
--
module Msg exposing (..)
import Global.Msg as Global
import TransactionList.Msg as TransactionList

type Msg
    = MsgForGlobal Global.Msg
    | MsgForTransactionList TransactionList.Msg

One of the things I like about this pattern is how readable each top level module is with import aliases.

View, Update, and Global State

The view and update functions compose similarly but I pass the top level model down to both so that they can cherry pick whatever state they need.

The lower level update functions can look at all the state and just return the piece of the model they are responsible for. For example the Global model can have common entities and state specific to the transaction list live in the TransactionList model.

Views are similar in that they can take state from the global model as well as their own model and render as necessary.

--
-- Update.elm
--
module Update exposing (..)

import Msg exposing (Msg)
import Model exposing (Model)
import Global.Update as Global
import TransactionList.Update as TransactionList

updateWithCmd : Msg -> Model -> ( Model, Cmd Msg )
updateWithCmd msg model =
    ( update msg model, updateCmd msg )

update : Msg -> Model -> Model
update msg model =
    { model
        | global = Global.update msg model
        , transactionList = TransactionList.update msg model
    }

updateCmd : Msg -> Cmd Msg
updateCmd msg =
    Cmd.batch
       [ TransactionList.updateCmd msg
       ]
       
--
-- View.elm
--
module View exposing (..)

import Model exposing (Model)
import Msg exposing (Msg)
import TransactionList.View as TransactionListView
import Html exposing (..)
import Html.Attributes exposing (..)

rootView : Model -> Html Msg
rootView model =
    div [ class "container" ] 
        [ TransactionListView.view model ]

This approach seems to be working pretty well so far and it seems like adding routing shouldn’t be too difficult.