Hello Elixir. Wow.

September 03 2015 Fun Learning Elixir

I don't know anything about elixir but I very much want to learn it. I like learning new things - I feel it's required for our industry. It's easy to feel a twinge of snark about this, I feel that too, every day. But every now and again something comes along and I just find myself getting pulled right in.

I know nothing about Elixir short of 6 chapters I read in a book over the weekend and some goofing around today. If you want to learn along with me, hurrah! Let's go.

Why Is Elixir Exciting?

For me it's both a positive and a negative reason. I remember the joy I felt when Ruby/Rails hit the scene, dragging me away from .NET. Five or so years later, Node came along and made life so much simpler - moving away from the monolithic web monster that's so easy to create with Rails.

But Node is Javascript, and to be honest my programming skills have eroded tremendously since I started doing Node full time over the last 4 years or so. I found this out when I tried to write some C# a few weeks ago. Ugh.

C# introduced me to some very interesting programming concepts, which I further exercised with Ruby and then flat-out ditched while working with JavaScript. Sigh.

Elixir is exciting to me because it's fast, fast, my god it's fast and has many of the nice facilities that Ruby does - with the power of Erlang behind it.

I like this.

Can I Build Something Right Now?

For most languages that you're learning the answer is a flat "no". Getting set up takes some time (Java, C#, Haskell, etc). With Elixir you just install it and you can write some code:

brew update && brew install elixir

This is installing elixir using Homebrew on a Mac. other installation options are here

Within 30 seconds you have Elixir (and Erlang) on your machine. You can see this right now:

iex

That opens up the Elixir REPL. Type something:

iex> x = "Rob"
Rob
iex> x
Rob

You just wrote some Elixir. Nicely done! For me that's the first victory - 0 to code in under 5 minutes. Nice!

Mix, NPM For Elixir

Sort of. Mix is a "project and build" manager for Elixir. It's a little bit of Rake, a little NPM - kind of in-between. Mix will create a new project for you - which you can think of as both a package and an executable (this is a command line command, not an iex command):

mix new hello_elixir
* creating README.md
* creating .gitignore
* creating mix.exs
* creating config
* creating config/config.exs
* creating lib
* creating lib/hello_elixir.ex
* creating test
* creating test/test_helper.exs
* creating test/hello_elixir_test.exs

This is so, so nice. It's created a tight little package structure for us, with a README, a .gitignore, a test directory (ready for tests!) and what's obviously an entry point with a mix.exs file. Even a central place for "config stuff"!

This is brilliant. It's also heavily commented so you can read through and have a basic understanding of what's going on.

Something Useful

If you want to skip right ahead - go for it. I put all the code on Github and will be tweaking it over the coming weeks.

So I could step through and do some silly crap where I output "Hello World" - but I'd rather do something more fun. How about we connect to a database and execute a query. Great idea - let's create a new Mix project called "Membership":

mix new membership
* creating README.md
* creating .gitignore
* creating mix.exs
* creating config
* creating config/config.exs
* creating lib
* creating lib/membership.ex
* creating test
* creating test/test_helper.exs
* creating test/membership_test.exs

I want to create a harness around my pg_auth project. It's a set of Postgres tables, functions, etc for handling membership in your application. I want to see how hard it is to execute a query, and WTF is going on here.

Yes I could read the book more, but I much prefer to actually do something, learning as I go.

The first thing to find out is whether there's a PostgreSQL driver. Elixir has been around for a while, so I'm fairly certain there is one, and yes indeed there is.

Now, I need to figure out how to get that project into my new project. The instructions are right on the main page of the Postgrex project: so open up mix.exs and edit the dependencies as well as which applications we'll be using inside our own:

def application do
  [applications: [:logger, :postgrex]]
end

#...

defp deps do
  [{:postgrex, "~> 0.9.1"}]
end

The bottom directive says that we want the postgrex package, the top directive says we want to "mount it" in our app... I think. Still a bit hazy on what exactly is going on here.

The next step is to make sure the dependencies are installed:

mix deps.get

This goes out to "hex.pm" - kind of like npmjs.org - it's where the package bits for Elixir are stored online. It will look for the postgrex package (version 0.9.1) and download it - creating a new directory called deps in our project.

Have a look in there (after you run this command). It's the source. Which I think is quite nice. It is, essentially, NPM's "nodemodules" approach - grab the source and stick it in your project so you can review etc. This seems to be a single-level dependency graph, unlike nodemodules, which recursively grabs the entire internet just in case.

Anyway - we have what we need here - access to a database.

The Membership Module

Let's write some code. I have a function called "membership.register" that I want to call, and I want to return the results somehow. For this I'll crack open membership_test.ex and have a look at the test structure:

defmodule MembershipTest do
  use ExUnit.Case

  test "the truth" do
    assert 1 + 1 == 2
  end
end

Gloriously sparse. It's not hard to figure out what's going on here - especially if you know Ruby. We're defining a module with a do block (yay!), bringing in some help from the ExUnit test library, and then writing a wonderfully terse test.

The fun Ruby vibes are setting in. This is exciting.

Let's write our test and watch as everything explodes:

defmodule MembershipTest do
  use ExUnit.Case

  test "Registration succeeds with valid credentials" do
    {:ok, res} = Membership.register({"test@test.com", "password"})
    assert res.success,res.message
  end

end

This is stretching what I know about Elixir - but basically I have a tuple on the left there, that's being "matched" with the result of the Membership.register/1 on the right.

We're now in the new language weeds. Here's what I know so far about this.

Every function in Elixir works on the concept of "pattern matching". You don't just call a function or "send a message" as you do in Ruby (though those ideas work too). You try to match things on both the left and right side of the assignment.

On the left I have a tuple with the first element being an "atom". You can think of this exactly as a Ruby symbol. The second element is any kind of data coming from the function result. This is a different kind of thing for me, but I think I get it and I'm going to punt on talking about that more because, simply, I'm just not sure what I can really do with it.

OK, let's write our register routine:

defmodule RegistrationResult do
    defstruct success: false, message: nil, new_id: 0
end

defmodule Membership do

  def register({email, password}) do
    {:ok, pid} = Postgrex.Connection.start_link(database: "bigmachine")
    sql = "select * from membership.register($1, $2);"
    {:ok, res} = Postgrex.Connection.query(pid, sql, [email, password])
    [record | last] = res.rows
    [new_id, validation_token, auth_token, success, message] = record
    {:ok, %RegistrationResult{success: success, message: message, new_id: new_id}}
  end

end

This is going to look very, very strange to you if you've never seen Elixir. It might also look incredibly strange to you even if you have! I'm very new to this stuff.

In the first lines up there I'm defining a struct to hold my result. I don't need to do this, but it's a nice way to package up results. I could use a Dict, Hash, or Map (or even a tuple) - but I like working with dot notation and you can do that with a struct.

In the register function I'm accepting a tuple (which is a good thing to do - a single argument that is flexible and expandable) and then opening the connection, matching it against a tuple to hold the pid.

That pid is not an operating system pid, it's Process pid built into elixir which is pretty mind-twisty. It's like having a little message queue all your own right inside your runtime. I don't know nearly enough to talk more about it, but I'm excited to get there.

OK, the next lines are pretty clear - I create a SQL string, pass the params, and execute using a pattern match on {:ok, res} where res is a struct for handling results.

Now that I have the result, I need to peel off the "head" of the rows array. This is where I think my code is pretty damn messy and I'm sure there's some better way - but for the sake of learning and trying to get something done I hacked the crap out of it.

The deal is that you can ask for the "head" and "tail" of a list by using this notation:

[h | t] = [1,2,3,4]

Here, h will equal "1" and t will equal "[2,3,4]". You might be thinking "WTF? Why?" like I did, then you get to see some recursive action and it will blow your mind. Again: I need to know more about that before I start writing anything.

So, the record I want is the first element of the return array - that means I need to pattern match against a list of variables, which will hold those values. Weird notation, but I think it's pretty interesting.

Finally I kick up a new struct and assign the results, making sure to pass the :ok atom first thing.

Testing This Properly

This took me a while to figure out. I wanted to write a test that executed the register function once so I could write a set of asserts on the result - I don't want to call register multiple times.

I tried my Ruby approach, using module-level variables and assigning them in the before block, which didn't work. Turns out it's much simpler:

defmodule MembershipTest do
  use ExUnit.Case

  setup do
    {:ok, pid} = Postgrex.Connection.start_link(database: "bigmachine")
    Postgrex.Connection.query!(pid, "delete from membership.users", [])
    {:ok, res} = Membership.register({"test@test.com", "password"})
    {:ok, [res: res]}
  end

  test "Registration with valid credentials", %{res: res} do
    assert res.success,res.message
  end

end

There's a setup "macro" (don't know what those are yet) that does what you expect. I open a connection, drop the users, and then run my registration function.

This is where things get neat. Every setup block can pass along a "context" to each test. It's just the result of setup - which is the very last line. The result has to be in the form of something like this:

{:ok}
{:ok, [key: value, another_key: another_value]} #a Dict

As you can see, Elixir has the convention of returning an atom in the fist place of a tuple to "define" what the tuple represents. If things are all good, :ok is returned. You can use anything in the first position - like :person, :refund, or :err. It's simply a convention - just like Node's callback structure (err, res, next).

Where I got into trouble was trying to send back a raw result (my RegistrationResult struct) from the register function - I kept getting an error about my RegistrationResult:

** (Protocol.UndefinedError) protocol Enumerable not implemented for %RegistrationResult

I got this error because the setup routine was trying to treat it like a Dict (dictionary) - which is enumerable (that's what Enum is, an enumeration module).

OK anyway I finally figured out I could send the result directly to my tests by using this:

    {:ok, res} = Membership.register({"test@test.com", "password"})
    {:ok, [res: res]}

That meant I could structure my test this way:

  test "Registration with valid credentials", %{res: res} do
    assert res.success,res.message
  end

And it worked. My god it worked.

Why I Put Wow In The Title

Elixir is intimidating to me. I suck as a programmer and really, I'm kind of a hack. But in about 3 hours I was able to take what I read in a book, connect to a database without a framework and execute something in a rather elegant way.

Wow. This made me quite happy today. We'll see about tomorrow.

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