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Tutorial: Introduction To Elm (Part 1)

Tutorial: Introduction To Elm (Part 1)

#theNewHotness

I've been hearing about the awesomeness of elm for a little while now. It's a strongly typed functional language written for crafting user interfaces that compiles into javascript. It also has a virtual-dom implementation that claims to be faster than react.

If you're keeping up with the hot buzzwords of the moment, we have:

  • Functional Reactive Programming
  • Virtual Dom
  • Strongly Typed Javascript
  • A new framework
  • A new package manager

What's not to love. It's been a while since I've re-written all of my javascript. So let's get what we can hack together!!

Our Test Project

To get started I'm replicating some code I hacked together a few afternoons ago for a client project. It's was such a simple task that I initially reached for jQuery. After a few refactorings, I was wishing I had just made some quick react components instead. It should provide a simple data driven example for testing our our budding elm skills.

Project Goal

I have a list of Insurance Carriers which each have an list of Insurance Reps for each carrier. We want to be able to choose a carrier and then a rep belonging to that carrier.

Initial Project Requirements

  • Create Two dropdowns: Insurance Carrier, Insurance Reps
  • The First dropdown should display the name of the Insurance Carrier and set the value to the carrier's id
  • The second dropdown should update when the first is changed, using only Reps belonging to the selected Insurance Carrier
  • The Insurance Rep dropdown should likewise show the Insurance Rep name and set the value to the Insurance Rep's id

Getting Started

For this tutorial, all you'll need is an empty directory. You can also download the git repo if you want to follow along. Each following post has it's own branch so you can start from same point I am as I write this.

Also, sure you have elm installed on your system. If you don't, you can follow the instructions here.

Putting Elm in the Project

Install the elm core library:

$ elm package install
---------------------------------------------------------
Some new packages are needed. Here is the upgrade plan.

  Install:
    elm-lang/core 3.0.0

Do you approve of this plan? (y/n) y  
Downloading elm-lang/core  
Packages configured successfully!  

Install libraries for working with the DOM:

$ elm package install evancz/elm-html
---------------------------------------------------------
To install evancz/elm-html I would like to add the following  
dependency to elm-package.json:

    "evancz/elm-html": "4.0.2 <= v < 5.0.0"

May I add that to elm-package.json for you? (y/n) y

Some new packages are needed. Here is the upgrade plan.

  Install:
    evancz/elm-html 4.0.2
    evancz/virtual-dom 2.1.0

Do you approve of this plan? (y/n) y  
Downloading evancz/elm-html  
Downloading evancz/virtual-dom  
Packages configured successfully!  

Now A Quick Hello World

First we'll create and compile our elm module file

# InsuranceDropdown.elm
module InsuranceDropdown where  
import Html

main =  
  Html.text "Show Me the Money!!!"

We'll go over the salient parts in a moment. But first, let's try to compile this and view it in the browser.

$ elm make InsuranceDropdown.elm --output index.html

If all goes well, you should see an index.html file with our text inside a div.

Here's what we've done so far:

  • We created a module to contain our code with module InsuranceDropdown where
  • We've created a .elm file with the same name as our module
  • We imported the Html library import Html
  • We created the function main that runs all of our code
  • We called the text function form the Html module which created the dom for us

Kick Up the Server

Compiling for every change can be lame. Before we start writing our app in earnest lets fire up elm reactor in the root of our project directory.

$ elm reactor
---------------------------------------------------------
elm reactor 0.16.0  
Listening on http://0.0.0.0:8000/  

This gives us a listing of the project files. If we click on the file, it will automatically recompile when we refresh the page. Also, by clicking on the wrench icon, we get a time-travelling debugger. So cool!

And NOW! … some html

For the purpose of this demo we'll use the Twitter Bootstrap convetion. Here's what we're aiming for:

 .row
  .col-sm-6
    select#InsuranceProviderId
      option
      ...
  .col-sm-6
    select#InsuranceRepId
      option
      ...

First, let's take a look at the documentation. According to the docs, here's how the basic tag function works:

div : List Attribute -> List Html -> Html  

So we'll write the name of the tag (an elm function), pass in a list of Attribute objects, a list of Html objects, and get an Html object back.

A basic tag would look like:

Html.div [Html.Attributes.class "container"] []  

Calling the Html module and the Html.Attributes module is going to get tiring after a while. We can change our imports so that any functions exposed in these modules can be called without the module name. This will give use something that looks much more like html.

import Html exposing(..)  
import Html.Attributes exposing (..)  

Now we can write the following:

div [class "container"] [  
  div [class "col-sm-6"] [],
  div [class "col-sm-6"] []
  ]

and we should get a .container div and a couple of other divs inside it. Let's apply this to the rest of our code and mock up a our dropdowns.

# InsuranceDropdown.elm
module InsuranceDropdown where

import Html exposing(..)  
import Html.Attributes exposing (..)

main : Html  
main =  
  div
      [ class "row"]
      [ div [class "col-sm6" ]
          [select [id "InsuranceProviderId"]
            [ option [value "1"] [text "Carrier 1"],
              option [value "2"] [text "Carrier 2"],
              option [value "3"] [text "Carrier 3"],
              option [value "4"] [text "Carrier 4"]
            ]
          ],
        div [ class "col-sm6" ]
          [select [ id "InsuranceCarrierId"]
            [ option [value "1"] [text "Rep 1"],
              option [value "2"] [text "Rep 2"],
              option [value "3"] [text "Rep 3"],
              option [value "4"] [text "Rep 4"]
            ]
          ]
      ]

What's Up Next

At this point we have:

  • Installed elm
  • played with the REPL
  • Looked at the local server
  • compiled our elm code
  • Make a quick mockup for our application code

In the next tutorial, we will make our dropdowns dynamic, read in some data and add the functionality to make them display data as they change.

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