An immutable chunk of data representing a DOM node. This can be HTML or SVG.

Just put plain text in the DOM. It will escape the string so that it appears exactly as you specify.

text "Hello World!"

Create a DOM node with a tag name, a list of HTML properties that can include styles and event listeners, a list of CSS properties like color, and a list of child nodes.

import Json.Encode as Json


hello : Node msg
hello =
  node "div" [] [ text "Hello!" ]


greeting : Node msg
greeting =
  node "div"
    [ property "id" (Json.string "greeting") ]
    [ text "Hello!" ]

When using HTML and JS, there are two ways to specify parts of a DOM node.

1. Attributes — You can set things in HTML itself. So the class in <div class="greeting"></div> is called an attribute.

2. Properties — You can also set things in JS. So the className in div.className = 'greeting' is called a property.

So the class attribute corresponds to the className property. At first glance, perhaps this distinction is defensible, but it gets much crazier. There is not always a one-to-one mapping between attributes and properties! Yes, that is a true fact. Sometimes an attribute exists, but there is no corresponding property. Sometimes changing an attribute does not change the underlying property. For example, as of this writing, the webkit-playsinline attribute can be used in HTML, but there is no corresponding property!

Create arbitrary properties.

import JavaScript.Encode as Json


greeting : Html
greeting =
    node "div" [ property "className" (Json.string "greeting") ] [
      text "Hello!"
    ]

Notice that you must give the property name, so we use className as it would be in JavaScript, not class as it would appear in HTML.

Create arbitrary HTML attributes. Maps onto JavaScript’s setAttribute function under the hood.

greeting : Html
greeting =
    node "div" [ attribute "class" "greeting" ] [
      text "Hello!"
    ]

Notice that you must give the attribute name, so we use class as it would be in HTML, not className as it would appear in JS.

Would you believe that there is another way to do this?! This corresponds to JavaScript's setAttributeNS function under the hood. It is doing pretty much the same thing as attribute but you are able to have "namespaced" attributes. This is used in some SVG stuff at least.

Specify a list of styles.

myStyle : Property msg
myStyle =
  style
    [ ("backgroundColor", "red")
    , ("height", "90px")
    , ("width", "100%")
    ]


greeting : Node msg
greeting =
  node "div" [ myStyle ] [ text "Hello!" ]

Create a custom event listener.

import Json.Decode as Json


onClick : msg -> Property msg
onClick msg =
  on "click" (Json.succeed msg)

You first specify the name of the event in the same format as with JavaScript’s addEventListener. Next you give a JSON decoder, which lets you pull information out of the event object. If the decoder succeeds, it will produce a message and route it to your update function.

Same as on but you can set a few options.

Options for an event listener. If stopPropagation is true, it means the event stops traveling through the DOM so it will not trigger any other event listeners. If preventDefault is true, any built-in browser behavior related to the event is prevented. For example, this is used with touch events when you want to treat them as gestures of your own, not as scrolls.

Everything is False by default.

defaultOptions =
    { stopPropagation = False
    , preventDefault = False
    }

This function is useful when nesting components with the Elm Architecture. It lets you transform the messages produced by a subtree.

Say you have a node named button that produces () values when it is clicked. To get your model updating properly, you will probably want to tag this () value like this:

type Msg = Click | ...


update msg model =
  case msg of
    Click ->
      ...


view model =
  map (\_ -> Click) button

So now all the events produced by button will be transformed to be of type Msg so they can be handled by your update function!

A performance optimization that delays the building of virtual DOM nodes.

Calling (view model) will definitely build some virtual DOM, perhaps a lot of it. Calling (lazy view model) delays the call until later. During diffing, we can check to see if model is referentially equal to the previous value used, and if so, we just stop. No need to build up the tree structure and diff it, we know if the input to view is the same, the output must be the same!

Same as lazy but checks on two arguments.

Same as lazy but checks on three arguments.

Works just like node, but you add a unique identifier to each child node. You want this when you have a list of nodes that is changing: adding nodes, removing nodes, etc. In these cases, the unique identifiers help make the DOM modifications more efficient.

The most generic way to create a Program. It is the primitive behind things like beginnerProgram and program in the Html.App module. Read about it there if you'd like to learn more about this.