Elm - Messages

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Message is a component in the Elm architecture. These components are generated by the View in response to the user's interaction with the application's interface. Messages represent user requests to alter the application's state.

Syntax 
--Message Syntax
type Message = some_message1 |some_message2 ...|some_messageN

llustration

The following example is a simple counter application. The application increments and decrements the value of a variable by 1 when the user clicks on the Add and Subtract buttons respectively.

The application will have 4 components. The components are described below −

Message

The messages for this example will be −

type Message = Add | Subtract

Model

The model represents the state of the application. In the counter application the model definition is given below; the initial state of counter will be zero.

model = 0

View

The view represents the visual elements of the application. The view contains two buttons ( + ) and ( - ) . The messages Add and Subtract are generated by the View when the user clicks on the + and - buttons respectively. The modified value of the model is then displayed by the View.

view model =
-- invoke text function
h1[]
[   div[] [text "CounterApp from Howcodex" ]
   ,button[onClick Subtract] [text "-"]
   ,div[][text (toString model)]
   ,button[onClick Add] [text "+"]
]

Update

This component contains code that should be executed for each message generated by the view. This is shown in the example below −

update msg model =
case msg of
Add -> model+1
Subtract -> model-1

Putting it all together

Step 1 − Create a folder MessagesApp and file MessagesDemo.elm

Step 2 − Add the following code in elm file −

import Html exposing (..)
import Html.Events exposing(onClick)

model = 0 -- Defining the Model

--Defining the View

view model =
   h1[]
   [  div[] [text "CounterApp from Howcodex" ]
      ,button[onClick Subtract] [text "-"]
      ,div[][text (toString model)]
      ,button[onClick Add] [text "+"]
   ]

--Defining the Messages

type Message = Add | Subtract

--Defining Update

update msg model =
case msg of
   Add -> model+1
   Subtract -> model-1

-- Define the main method
main =
   beginnerProgram
   {
      model=model
      ,view=view
      ,update=update
   }

Step 3 − Execute the elm make command in terminal. The elm make command compiles the code and generates an HTML file from the .elm file created above.

C:\Users\dell\elm\MessagesApp> elm make .\MessageDemo.elm
Some new packages are needed. Here is the upgrade plan.

   Install:
      elm-lang/core 5.1.1
      elm-lang/html 2.0.0
      elm-lang/virtual-dom 2.0.4

Do you approve of this plan? [Y/n] y
Starting downloads...

   ΓùÅ elm-lang/html 2.0.0
   ΓùÅ elm-lang/virtual-dom 2.0.4

ΓùÅ elm-lang/core 5.1.1
Packages configured successfully!
Success! Compiled 38 modules.
Successfully generated index.html

Step 4 − Open the index.html and verify the working as shown below −

elm make command
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