Hello, World

Hello, world

You can find all the code for this chapter here

It is traditional for your first program in a new language to be Hello, World.

  • Create a folder wherever you like

  • Put a new file in it called hello.exs and put the following code inside it

defmodule HelloWorld do
  def hello do
    IO.puts("Hello, World")
  end
end

HelloWorld.hello()

To run it execute elixir hello.exs.

Note that Elixir is a compiled language that runs on top of BEAM and in gereal mix build tool is used to compile and test Elixir code.

However Elixir allows to run code as a "script language". In this mode you need to name your file with .exs extension to be able to execute code in-demand. Is important to understand that difference between "compiled" and "scripted" mode is that in the first case code is compile directly to BEAM bytecode and can be reused/loaded anytime. On the other hand, in script mode code is compiled everytime you run the file as result is discarded.

How it works

When you write a program in Elixir and want to define named functions, you need to define a module first.

The defmodule keyword (actually it's a macro, but that will be in a future chapter) is how you define a module with a name that follows PascalCase convention and a do-end block that represents the module body. Think in a module as a way to group several co-related functions!

To define a function def keyword is used followed by it's name in snake_case and the function body wrapped in a do-end block.

So to print a text string we use puts/1 function from IO built-in module.

How to test

How do test this? An excellent code pattern is to separate "pure" code from the putside world side-effects. Your code/function is considered pure when it doesn't relies on, or modifies something outside its scope (eg. arguments).

The IO.puts/1 is a side-effect (printing to stdout) and the text string we sen in is our pure or "domain" code.

So let's separate these concers so it's easier to test (and maintain!)

defmodule HelloWorld do
  def print_hello do
    IO.puts(hello())
  end

  def hello do
    "Hello, world"
  end
end

Now create a new module HelloWorldTest in hello.exs where we are going to write a test for our HelloWorld module

# ... HelloWorld module

ExUnit.start(auto_run: false)

defmodule HelloWorldTest do
  use ExUnit.Case

  test "hello/0" do
    assert HelloWorld.hello() == "Hello, world"
  end
end

ExUnit.run()

Elixir has a built-in unit test framework called ExUnit which is a separate BEAM application that needs to be started mannually in this case.

Run elixir hello.exs in your terminal. It should've passed! Just to check, try deliberately breaking the test by changing the string content after the equal (==) operator.

In compiled mode and in a project set up with mix we would create a file called xxx_test.exs, where xxx is the module to be tested, and then execute mix test to run it. We will see more about mix in future chapters!

Notice how you have not had to pick between multiple testing frameworks and then figure out how to install. Everything you need is built in to the language and the syntax is the same as the rest of the code you will write.

Writing tests

Writing a test is just like writing a module, with a few additional steps:

  • The test module name should end with the word Test

  • ExUnit.Case module must be used, bringing its functionallity to the current module, so you can use "keywords" like test and assert

  • To check a truthy return value you should use assert

  • To check a falsy return value you should use refute

ExDoc

Another quality of life feature of Elixir is the documentation. You can generate your project documentation by using ex_doc official library.

The standard library has excellent documentation with examples. Navigating to "Kernel" documentation would be worthwhile to see what's available to you.

Hello, YOU

Now that we have a test we can iterate on our software safely.

In the last example we wrote the test after the code had been written just so you could get an example of how to write a test and define a function. From this point on we will be writing tests first.

Our next requirement is to let us specify the recipient of the greeting.

Let's start by capturing these requirements in a test. This is basic test driven development and allows us to make sure our test is actually testing what we want. When you retrospectively write tests there is the risk that your test may continue to pass even if the code doesn't work as intended.

To achieve this, let's crate a new test block inside HelloWorldTest module created before

test "hello/1" do
  assert HelloWorld.hello("Zoey") == "Hello, Zoey"
end

Now run elixir hello.exs, you should have a test failure like

  1) test hello/1 (HelloWorldTest)
     hello.exs:20
     ** (UndefinedFunctionError) function HelloWorld.hello/1 is undefined or private. Didyou mean one of:

           * hello/0

     code: assert HelloWorld.hello("Zoey") == "Hello, Zoey"
     stacktrace:
       HelloWorld.hello("Zoey")
       hello.exs:21: (test)



Finished in 0.00 seconds (0.00s async, 0.00s sync)
2 tests, 1 failure

Randomized with seed 215806

Although Elixir is a compiled language there's no statically type inference so the compiler tries to run tests and fails.

Now it's the perfect momento to understand why we're using function/x notation. Elixir allows developers to implement multi clause functions with same or different arguments arity.

This means that hello/0 represents a function named "hello" that receives no arguments meanwhile hello/1 is a function also named "hello" but receives only one argument. Note that there's no function overloading in Elixir. Actually the compiler understand hello/0 and hello/1 as different functions.

In this case, the compiler is teeling you that there's no "hello" function that recevies an argument defined in HelloWorld module.

So we have to add a new "hello" clause in our module that receives an argument (you can use another function name too)

def hello(name) do
  "Hello, world"
end

If you try and run your tests again your hello.exs last test will fail again because you're not using the given "name" argument at all.

  1) test hello/1 (HelloWorldTest)
     hello.exs:24
     Assertion with == failed
     code:  assert HelloWorld.hello("Zoey") == "Hello, Zoey"
     left:  "Hello, world"
     right: "Hello, Zoey"
     stacktrace:
       hello.exs:25: (test)

.

Finished in 0.00 seconds (0.00s async, 0.00s sync)
2 tests, 1 failure

Randomized with seed 840714

Let's make the test pass by using the "name" argument and concatenate it with Hello,

def hello(name) do
  # "Hello, #{name}"
  "Hello, " <> name
end

To concatenate text string we use <>/2 operator (see, operators in Elixir are only functions or macros with their own arity!).

We can also use string interpolation with Hello, #{name} syntax expression to insert "name" value into an existing string!

When you run the tests they should now pass. Normally as part of the TDD cycle we should now refactor.

A note on source control

At this point, if you are using source control (which you should!) I would commit the code as it is. We have working software backed by a test.

I wouldn't push to main though, because I plan to refactor next. It is nice to commit at this point in case you somehow get into a mess with refactoring - you can always go back to the working version.

There's not a lot to refactor here, but we can introduce another language feature, module attributes.

Module attributes

Module attributes have two main roles in Elixir:

  1. To annotate somehow the module

  2. Work as constants

As annotations

defmodule HelloWorld do
  @moduledoc """
  Implement functions that returns greetings
  """

  @doc """
  Returns a static greeting

  ## Examples

      iex> HelloWorld.hello()
      "Hello, world"

  """
  def hello do
    "Hello, world"
  end
end

In this example, we define the module documentation by using the module attribute syntax. Note that we also used multi-line strings that can be wrapped in three double-quotes.

Another useful reserved attribute is @doc that can be used to write documentation for any public function.

Elixir promotes the use of Markdown to write readable documentation. We can access the documentation of any compiled module directly from the interactive shell

$ iex

iex> h IO.puts/1
...

We'll see more about the interactive shell (iex) in other chapter.

As "constants"

Another way to use module attributes is to make a value more visible or reusable

defmodule Example do
  @prefix "Mx. "

  def greeting(name) do
    "Hello, " <> @prefix <> name
  end
end

Do not add a new line between module attribute name and its value as the compiler will assume you're using the attribute rather than defining it

Module attributes are "private" values so they can't be accessed by other modules unless you define a function to "export" its value

defmodule Example do
  @service_uri URI.parse("https://integration.io")

  def service_uri do
    @service_uri
  end
end

Note that remote functions may be called when defining a module attribute. In this example we're using the URI.parse/1 function that validates the given url into a URI structure.

Functions that are defined in the same module of the attribute can't be called to define it as the module (and their functions) have not yet been compiled when the attribute is being defined.

First refactor

With this new knowledge we can refactor our HelloWorld module

defmodule HelloWorld do
  @english_hello_prefix "Hello, "

  def hello(name) do
    @english_hello_prefix <> name
  end
end

After refactoring, re-run your tests to make sure you haven't broken anything.

Hello, world... again

The next requirement is when our function is called with an empty string it defaults to printing "Hello, World", rather than "Hello, ".

Start by writing a new failing test in our HelloWorldTest module

describe "hello/1" do
  test "saying hello to people" do
    assert HelloWorld.hello("Zoey") == "Hello, Zoey"
  end

  test "when an empty string is supplied" do
    assert HelloWorld.hello("") == "Hello, world"
  end
end

Here we are introducing another tool in our testing arsenal, describe/2. Sometimes it is useful to group tests around a "thing" or their context and then have tests describing different scenarios.

A benefit of this approach is you can set up shared code that can be used in the other tests.

Now that we have a well-written failing test, let's fix the code, using an if.

defmodule HelloWorld do
  @english_hello_prefix "Hello, "

  def hello(name) do
    if name == "" do
      @english_hello_prefix <> "world"
    else
      @english_hello_prefix <> name
    end
  end
end

Good! If we run our tests we should see it satisfies the new requirement and we haven't accidentally broken the other functionality.

We learned on how to use an if/2 expression that consists in a condition for a truthy value and a do-block with an optional else branch. So if the condition pass (results to a truthy value) the code before else is executed. If condition evaluates to a falsy value the else branch code is executed.

When there isn't an else branch and the condition evaluates to falsy, then a nil value is returned.

Pattern matching

if/2 expressions can be really useful to determine control flow in our code, however Elixir also supports the use of Pattern matching.

To summarize, pattern matching can be used to check the structure of a given value. Let's take a look on our next refactor to better understanding

defmodule HelloWorld do
  @english_hello_prefix "Hello, "

  def hello("") do
    @english_hello_prefix <> "world"
  end

  def hello(name) do
    @english_hello_prefix <> name
  end
end

Here we are using the multi clause function technique again, but in the first clause the empty string value was "harded-coded". Why this can be better?

The "harded-coded" empty string actually is a pattern that we supply to the function definition, so the compiler will execute this function clause only if an empty string is given.

In the other hand, the second clause of hello/1 is executed every time a value that isn't an empty string is given, so is the "default" or "fallback" clause in this case.

This is only a try of pattern matching and we will be using it in several forms on future chapters!

Back to source control

Now we are happy with the code I would amend the previous commit so we only check in the lovely version of our code with its test.

Discipline

Let's go over the cycle again

  • Write a test

  • Make the compiler pass

  • Run the test, see that it fails and check the error message is meaningful

  • Write enough code to make the test pass

  • Refactor

On the face of it this may seem tedious but sticking to the feedback loop is important.

Not only does it ensure that you have relevant tests, it helps ensure you design good software by refactoring with the safety of tests.

Seeing the test fail is an important check because it also lets you see what the error message looks like. As a developer it can be very hard to work with a codebase when failing tests do not give a clear idea as to what the problem is.

By ensuring your tests are fast and setting up your tools so that running tests is simple you can get in to a state of flow when writing your code.

By not writing tests you are committing to manually checking your code by running your software which breaks your state of flow and you won't be saving yourself any time, especially in the long run.

Keep going! More requirements

Goodness me, we have more requirements. We now need to support a second parameter, specifying the language of the greeting. If a language is passed in that we do not recognise, just default to English.

We should be confident that we can use TDD to flesh out this functionality easily!

Write a test for a user passing in Spanish. Add it to the existing suite.

test "hello/2" do
  assert HelloWorld.hello("Yan", "Spanish") == "Hola, Yan"
end

Remember not to cheat! Test first. When you try and run the test, the compiler should complain because you are calling an undefined hello/2 function.

  1) test hello/2 (HelloWorldTest)
     hello.exs:38
     ** (UndefinedFunctionError) function HelloWorld.hello/2 is undefined or private. Did you mean one of:

           * hello/0
           * hello/1

     code: assert HelloWorld.hello("Yan", "Spanish") == "Hola, Yan"
     stacktrace:
       HelloWorld.hello("Yan", "Spanish")
       hello.exs:39: (test)

...

Finished in 0.00 seconds (0.00s async, 0.00s sync)
4 tests, 1 failure

Randomized with seed 376422

Define a new function hello/2 that receives a name and a language

defmodule HelloWorld do
  @english_hello_prefix "Hello, "

  def hello("", language) do
    @english_hello_prefix <> "world"
  end

  def hello(name, language) do
    @english_hello_prefix <> name
  end
end

When you try and run the test again it will fail on the grounds that we aren't using the language supplied argument

  1) test hello/2 (HelloWorldTest)
     hello.exs:46
     Assertion with == failed
     code:  assert HelloWorld.hello("Yan", "Spanish") == "Hola, Yan"
     left:  "Hello, Yan"
     right: "Hola, Yan"
     stacktrace:
       hello.exs:47: (test)

...

Finished in 0.00 seconds (0.00s async, 0.00s sync)
4 tests, 1 failure

Randomized with seed 721482

Here you can fix this failure using the if/2 expression... or try to use pattern matching again!

defmodule HelloWorld do
  @english_hello_prefix "Hello, "

  def hello("", "Spanish") do
    "Hola, world"
  end

  def hello("", _language) do
    @english_hello_prefix <> "world"
  end

  def hello(name, "Spanish") do
    "Hola, " <> name
  end

  def hello(name, _language) do
    @english_hello_prefix <> name
  end
end

OMG, now things are starting to complicate. We defined two more hello/2 clauses:

  • One that receives an empty string and also "Spanish" as language value

  • One that receives a name and also "Spanish" as language value

We also modified those old two clause prefixing the "language" argument with an underscore. That's a special syntax that tells to the compiler that this match expression can receive any value and we will ignore it as we don't need them.

The tests should now pass.

Now it is time to refactor. You should see some problems in the code, "magic" strings, some of which are repeated. Try and refactor it yourself, with every change make sure you re-run the tests to make sure your refactoring isn't breaking anything.

defmodule HelloWorld do
  @english_hello_prefix "Hello, "
  @spanish_hello_prefix "Hola, "

  def hello("", "Spanish") do
    @spanish_hello_prefix <> "world"
  end

  def hello("", _language) do
    @english_hello_prefix <> "world"
  end

  def hello(name, "Spanish") do
    @spanish_hello_prefix <> name
  end

  def hello(name, _language) do
    @english_hello_prefix <> name
  end
end

French

  • Write a test asserting that if you pass in "French" you get "Bonjour, "

  • See it fail, check the error message is easy to read

  • Do the smallest reasonable change in the code

You may have written something that looks roughly like this

defmodule HelloWorld do
  @english_hello_prefix "Hello, "
  @spanish_hello_prefix "Hola, "
  @french_hello_prefix "Bonjour, "

  def hello("", "Spanish") do
    @spanish_hello_prefix <> "world"
  end

  def hello("", "French") do
    @french_hello_prefix <> "world"
  end

  def hello("", _language) do
    @english_hello_prefix <> "world"
  end

  def hello(name, "Spanish") do
    @spanish_hello_prefix <> name
  end

  def hello(name, "French") do
    @french_hello_prefix <> name
  end

  def hello(name, _language) do
    @english_hello_prefix <> name
  end
end

case

Having several many different function clauses in general is not an actual problem, but in some cases you will want to preserve redability while reducing verbosity.

This can be achieved by using a case/2 expression to avoid adding two new more function clauses every time we want to support a new language.

defmodule HelloWorld do
  @english_hello_prefix "Hello, "
  @spanish_hello_prefix "Hola, "
  @french_hello_prefix "Bonjour, "

  def hello("", language) do
    case language do
     "Spanish" -> @spanish_hello_prefix <> "world"
     "French" -> @french_hello_prefix <> "world"
     _any -> @english_hello_prefix <> "world"
    end
  end

  def hello(name, language) do
    case language do
     "Spanish" -> @spanish_hello_prefix <> name
     "French" -> @french_hello_prefix <> name
     _any -> @english_hello_prefix <> name
    end
  end
end

Write a test to now include a greeting in the language of your choice and you should see how simple it is to extend our amazing function.

one...last...refactor?

You could argue that maybe our case/2 logic is being duplicated. The simplest refactor for this would be to extract out this functionality into another function.

defmodule HelloWorld do
  @english_hello_prefix "Hello, "
  @spanish_hello_prefix "Hola, "
  @french_hello_prefix "Bonjour, "

  def hello("", language) do
    greeting_prefix(language) <> "world"
  end

  def hello(name, language) do
    greeting_prefix(language) <> name
  end

  defp greeting_prefix("Spanish") do
    @spanish_hello_prefix
  end

  defp greeting_prefix("French") do
    @french_hello_prefix
  end

  defp greeting_prefix(_any) do
    @english_hello_prefix
  end
end

Here we defined a new function greeting_prefix/1 that receives a language with three different clauses that pattern matches with our languages possibilities and also implement a fallback.

Of course you can define this function in term of the case/2 expression if you prefer.

Wrapping up

Who knew you could get so much out of Hello, world?

By now you should have some understanding of:

Some of Elixir's syntax around

  • Writing and grouping tests

  • Defining functions, with arguments and multiple clauses

  • Basic usage of Pattern Matching

  • if/2 and case/2 expressions

  • Module attributes

The TDD process and why the steps are important

  • Write a failing test and see it fail so we know we have written a relevant test for our requirements and seen that it produces an easy to understand description of the failure

  • Writing the smallest amount of code to make it pass so we know we have working software

  • Then refactor, backed with the safety of our tests to ensure we have well-crafted code that is easy to work with

In our case we've gone from hello/0 clause to hello/1, to hello/2 in small, easy to understand steps.

This is of course trivial compared to "real world" software but the principles still stand. TDD is a skill that needs practice to develop, but by breaking problems down into smaller components that you can test, you will have a much easier time writing software.

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