Spotted Arrow

2017-04-23

Let’s Build a Random Quote Machine in Elm

Objectives

  • Learn about and implement the Elm architecture using messages, command messages, types, type aliases, and other related concepts.
  • Demonstrate how to utilize Elm community packages.
  • Create a simple Random Quote Machine using Ellie, the friendly Elm editor.

Editor

Resources

Elm is a functional programming language that compiles down to JavaScript. From my very little experience in using it thus far, it is a real joy, especially once you grasp the way data flows throughout the program. In this lesson, we will create a simple Random Quote Machine while highlighting the core concepts of the Elm architecture.

To begin let’s open up the friendly Elm editor, Ellie, in the browser of your choice.

What I like to do first is create a model that describes the shape of the data that I want to display in the browser. I want to display an author name and the author’s quote. In programmatic form it will look like this:

You probably notice type alias and are 🤔. A type alias describes a data set. Here I have an object that has keys author and quote, both as String data types. You have worked with data types before such as String, Bool, List, Array. Type aliases just allow you to extend the functionality of types to ensure that the data you pass around in your program is what you expected it to be. Just as in JavaScript you can’t call Number(5).slice(1) so too in Elm you cannot render the view without the correct model in place. If you wrote the above code in the Ellie editor and compile it, everything should still work as expected.

Now that I have the model type alias I will now create the model function. For now, it won’t do much except provide me with a default author and quote for me to render in the view. It will look a little something like this:

What is model: Model? All this is saying is that my model function returns a Model type which I described using the type alias. This is known as a function signature and it is quite helpful when trying to understand what a function is doing.

Now that I have the model let’s render a view with it. I will now create a view function that takes our model and then renders the author and quote. Pretty simple, huh? It will look a little something like this:

Again, I have a view function that takes a model and returns Html. It actually says Html Msg but I won’t be needing the Msg part till later. Because I am using Elm’s Html.beginnerProgram I will need an update function even though I am not using it at this point. For now, it’s not going to be doing very much.

Since I am just rendering the model in the view I am not doing anything special in the update. I created a Msg type of NoOp, no operation, so that I can have the correct function signature for the update function that Html.beginnerProgram expects.

To put it all together I am using the Html.beginnerProgram that I spoke of before. This wires everything up and makes our app come alive 😱!

If you did everything above and you hit “COMPILE” in the top left of the browser.

We have come a long way. Here is what the code looks like so far:

At this point, we have a quote that’s not random. You came to see a random quote machine. You must be so disappointed!

First I will create a list of quotes based on the type alias Model I made at the very beginning. I will call this function quotes.

What I have here is a List of Models. List is another data type in Elm. Lists are very similar to Arrays with some optimizations. From this list, we will randomly choose a quote. Now I can get into messages and how I will use messages to generate a random number and from that number pull a quote from the quotes List.

The first message type I will create is called FetchQuote. It will kick off the process of generating a random number. The other message type I need is NewQuote which will take an Int as a data type. These message types will look a little something like this:

What FetchQuote will do is create a Cmd Msg. Cmd Msg, or command message, is how the Elm architecture handles side-effects. Let’s see this in action in our update function.

I have just added a crazy amount of new stuff and now you are probably going to walk away and never read this post again, but hear me out! The update function now takes a Msg and a Model and returns a tuple of Model and Cmg Msg. You see, when I click on FetchQuote it will return the original model, no changes, but will create a side-effect of generating a random number. Random.generate creates a Cmd Msg and passes the newly generated number into NewQuote. When that happens the update function listens and pulls in the Cmd Msg and you can see that NewQuote now returns a new model but no Cmd Msg, that’s why it says Cmd.none. Now let’s dig into line 7. What is happening here are a few things. I am using a helper library called elm-community/list-extra and exposing the getAt function. This helps me to retrieve a quote at the randomly created index from the quotes list. What is all that other stuff? The getAt function returns a Maybe, essentially telling us that if the value exists at that index, it will return Just and that value. Otherwise, it will return Nothing. This prevents us from breaking our application in the case that an index does not exist in the list. Because of this, we have to extract out the value from the Maybe using withDefault which comes from the Maybe module. The withDefault function takes a default value and the actual value if it exists. I created a function called defaultQuote to be our default in case the index does not exist, but because we are only generating numbers 0 thru the length of our list, chances are you will not see the defaultQuote (unless you up the value range of the random number). Just as a test, try out making the defaultQuote function on your own. Hint: it looks just like the model.

Now that we have all that in place, I need a button to kick this whole process off. I will update the view function to now call the FetchQuote message inside of a button. My new view function will look like this.

There are a few more steps before this all works. I need to update the Elm program we are running to be Html.program and not Html.beginnerProgram. Also because we leveled up we need to add subscriptions, which in our case will do nothing. In Html.program, there is no model but instead, init. The init function returns a tuple of a Model and a Cmd Msg. I need to pull in a library I mentioned earlier as well. If you look on the left pane of the editor you will see a button “ADD A PACKAGE.” Click on that button and a search box will pop up. If you type in “list extra” the package elm-community/list-extra should appear at the top of your search results. Click on the “INSTALL” button that appears to the right of the package name. Lastly, don’t forget your defaultQuote or your imports!

If you are getting errors because you have compiled but don’t have all the correct imports/libraries, that’s okay. I actually want you to see those errors because they are pretty helpful and a core part of what makes Elm great, at lease in my eyes.

If you are feeling confident at this point I would click “COMPILE.” You may get some errors but I find Elm errors to be quite pleasant and I believe in you that you can navigate them with ease. If you are not up for those pesky errors here is a gist of the complete code:

And if you want to see it all in action, here it is in the friendly editor Ellie:

Wow, if you made it this far, thank you! You deserve a gold 🌟!

In this lesson, I covered the Elm architecture and how to stand up a project that has dynamic content. I pulled in community packages and in the end, created a Random Quote Machine that is pretty neat if I do say so myself 😃.

I hope this lesson was helpful, let me know in the comments below 👇🏼!