‘Iacta alea est’
Inherent in the Elm language are the strict constraints placed on programs by the features immutibility and type inference. These naturally derive what is called The Elm Architecture, consisting of three parts:
Let’s work our way up to the Counter example, which is the starting point for the Elm Architecture Tutorial. In this post we start with the View.
Technically we don’t need an interactive application if all we want to do is display something on the screen (view). That means we can leave out our state (model) and any interactivity (update). Let’s make a simple application that displays the text “Hello Elm!”.
We declare a
main function that renders a string to an HTML document as a text node. For this we will need to import the text function from the Html package:
import Html exposing (text) main = text "Hello Elm!"
Have a go yourself with the Elm Online Editor. Type the code in and hit the ‘Compile’ button. You’ve written your first Elm program!
Note that the
main function is picked up by the Elm runtime as the entry point to your application and it kicks everything off.
A text node works, but we can move onto HTML elements. Let’s put our text in a
import Html exposing (text, div) main = div [ ] [ text "Hello Elm!" ]
We’ve had to expand our import statement to include the
div function and our
div function (like all element node functions) takes two arguments, both of type List. The first list, which is empty in our case, is an attribute list, of type Attribute. The second argument is a list of children of type Html. The function then returns something of type Html.
Elm uses Type Annotations, so the type annotation for
div looks like this:
div : List Attribute -> List Html -> Html
which is a much more concise way of what I wrote above in long hand.
The following illustrates a fuller example with a more nested structure:
import Html exposing (..) import Html.Attributes exposing (class, href) main = div [ class "content" ] [ p [ ] [ text "Check this site out: " , a [ href "http://elm-lang.org/" ] [ text "Elm" ] ] ]
We’ve opened up the
Html import to include all functions with
.. and pulled in the
Html.Attributes module to give us access to the
href functions. If you have used the Haml or Jade templating engines before, you’ll spot the resemblance in syntax and structure.
What’s really nice is that we can start to break up that example into smaller parts and compose it back into the whole. This is where the functional nature of Elm allows us to do with our ‘template’ what we do with our code—because it is code.
import Html exposing (..) import Html.Attributes exposing (..) linkToElm = a [ href "http://elm-lang.org/" ] [ text "Elm" ] content link = p [ ] [ text "Check this site out: " , link ] main = div [ class "content" ] [ content linkToElm ]
Although this example is a little contrived, you can see how easy it would be to break up more complex sections of UI to keep things neat and tidy.
The last optimization we can make to better perceive the Elm Architecture is to name a
view function which is in turn called by
main. Our first, simplest example would therefore look like this:
import Html exposing (text) view = text "Hello Elm!" main = view
Tomorrow we’ll take a look at integrating the Model into what we’ve done today with the View.
—Wednesday 11th November 2015