Level 2.5 - TEA TIME

The Elm architecture is made up of three parts: model, view and update.

Model

The model is the state of our application. The application state can consist of for example data fetched from an API, which page the user is currently viewing, the current state of an input form etc. In Elm this is usually modeled with a record, defined by using a type alias. For example:

type alias Model =
    { userList : List User
    , currentPage : Page
    , nameInput : String
    }

View

A function that takes the current application state as an argument and returns some HTML.

Update

A function that takes a "message", the current application state and returns an updated application state. This message is modeled with a union type, so usually the update function consists of pattern matching on the message value.

Messages are events that happen in our application. These could be for example:

• User clicked the cancel button
• User clicked the exit button
• User entered some text in a text field
• We received some data from an API

Browser.sandbox

Browser.sandbox is the simplest way to run an Elm application. It takes only one argument; a record with a field for the initial application state (init), a field for the update function, and a field for the view function. With this, the Elm runtime will make sure that when there's some user-input in your application, the message will be sent to your update function, and when you have updated your model it will be sent to the view function so the HTML gets updated.

Let's examine Brower.sandbox's type signature.

Browser.sandbox
    : { init : modelType
      , view : modelType -> Html msgType
      , update : msgType -> modelType -> modelType }
    -> Program () modelType msgType

Here we can see that because of Elm's static typing these three fields must "match" each other:

• The type of the initial model must match the type of the argument to the view function.
• The type of messages returned from view (the msgType part in Html msgType) must be the same type as the first argument to update
• The type of the second argument to update must match the type of the model

Browser.sandbox returns a Program () modelType msgType but you can forget about the () modelType msgType part for now. The important part is that it returns a Program , which is just what the Elm runtime wants.

Html msgType

This part can seem a bit confusing, so let's look at lists first. A list in Elm has the type List someType. This means that it is a list where the elements are of type someType. For example:

• List Int is a list of ints
• List String is a list of strings

The concept is the same for Html msgType. It says that the HTML "contains" some msgType; all messages that come from this bit of HTML will be of type msgType. For example:

• Html Int - messages from this HTML will be ints
• Html String - messages from this HTML will be string

In practice we will use union types for our messages as they are incredibly useful, but it is useful to realize that our messages could in principle be of any type.

In the end, this means that when you hook up your view and update functions using Browser.sandbox, the messages sent to update will be of the type contained in the HTML.

Exercise time

To practice using the Elm architecture we will create the "Hello, world" of interactive web applications; a counter app. It will consist of one button for incrementing a number, one button for decrementing a number and a <span> for showing the number.

You can use the following skeleton code to get quickly up and running:

module Main exposing (main)

import Browser
import Html exposing (..)

main =
    Browser.sandbox
        { init = 0
        , update = update
        , view = view
        }

type alias Model
    = Int

type Msg
    = NoOp

update : Msg -> Model -> Model
update msg model =
    model

view : Model -> Html Msg
view model =
    text "Hello, I'm a view"

Task:

1. Implement the view function. It should output the following HTML (substitute 0 for the value of the model):

<div>
    <button>-</button>
    <span>0</span>
    <button>+</button>
</div>

1. Create messages for incrementing and decrementing the model
   • These should be constructor functions for the Msg type
   • When you have done this, use pattern matching on the value of msg (case msg of ...) in update.
2. Add onClick handlers on the two buttons
   • onClick is a function from the Html.Events module. Import it by adding import Html.Events exposing (onClick) at the top of your file
   • It has the type onClick : msgType -> Attribute msgType. Remember also that the second argument of HTML nodes (button in our case here) is of type List (Attribute msgType)

When you have done this, you should be able to increment and decrement the number by clicking on the buttons!

Bonus level:

Can you modify the program so that you have buttons for incrementing/decrementing by 1, 5 and 10 without using more than two different messages?

Hint: constructor functions can take arguments