Modules

In Elixir we group several functions into modules. We’ve already used many different modules in the previous chapters such as the String module:

iex> String.length("hello")
5

In order to create our own modules in Elixir, we use the defmodule macro. We use the def macro to define functions in that module:

iex> defmodule Math do
...>   def sum(a, b) do
...>     a + b
...>   end
...> end

iex> Math.sum(1, 2)
3

In the following sections, our examples are going to get longer in size, and it can be tricky to type them all in the shell. It’s about time for us to learn how to compile Elixir code and also how to run Elixir scripts.

Compilation

Most of the time it is convenient to write modules into files so they can be compiled and reused. Let’s assume we have a file named math.ex with the following contents:

defmodule Math do
  def sum(a, b) do
    a + b
  end
end

This file can be compiled using elixirc:

$ elixirc math.ex

This will generate a file named Elixir.Math.beam containing the bytecode for the defined module. If we start iex again, our module definition will be available (provided that iex is started in the same directory the bytecode file is in):

iex> Math.sum(1, 2)
3

Elixir projects are usually organized into three directories:

  • ebin - contains the compiled bytecode
  • lib - contains elixir code (usually .ex files)
  • test - contains tests (usually .exs files)

When working on actual projects, the build tool called mix will be responsible for compiling and setting up the proper paths for you. For learning purposes, Elixir also supports a scripted mode which is more flexible and does not generate any compiled artifacts.

Scripted mode

In addition to the Elixir file extension .ex, Elixir also supports .exs files for scripting. Elixir treats both files exactly the same way, the only difference is in intention. .ex files are meant to be compiled while .exs files are used for scripting, without the need for compilation. For instance, we can create a file called math.exs:

defmodule Math do
  def sum(a, b) do
    a + b
  end
end

IO.puts Math.sum(1, 2)

And execute it as:

$ elixir math.exs

The file will be compiled in memory and executed, printing “3” as the result. No bytecode file will be created. In the following examples, we recommend you write your code into script files and execute them as shown above.

Named functions

Inside a module, we can define functions with def/2 and private functions with defp/2. A function defined with def/2 can be invoked from other modules while a private function can only be invoked locally.

defmodule Math do
  def sum(a, b) do
    do_sum(a, b)
  end

  defp do_sum(a, b) do
    a + b
  end
end

IO.puts Math.sum(1, 2)    #=> 3
IO.puts Math.do_sum(1, 2) #=> ** (UndefinedFunctionError)

Function declarations also support guards and multiple clauses. If a function has several clauses, Elixir will try each clause until it finds one that matches. Here is an implementation of a function that checks if the given number is zero or not:

defmodule Math do
  def zero?(0) do
    true
  end

  def zero?(x) when is_integer(x) do
    false
  end
end

IO.puts Math.zero?(0)         #=> true
IO.puts Math.zero?(1)         #=> false
IO.puts Math.zero?([1, 2, 3]) #=> ** (FunctionClauseError)
IO.puts Math.zero?(0.0)       #=> ** (FunctionClauseError)

Giving an argument that does not match any of the clauses raises an error.

Similar to constructs like if, named functions support both do: and do/end block syntax, as we learned do/end is just a convenient syntax for the keyword list format. For example, we can edit math.exs to look like this:

defmodule Math do
  def zero?(0), do: true
  def zero?(x) when is_integer(x), do: false
end

And it will provide the same behaviour. You may use do: for one-liners but always use do/end for functions spanning multiple lines.

Function capturing

Throughout this tutorial, we have been using the notation name/arity to refer to functions. It happens that this notation can actually be used to retrieve a named function as a function type. Start iex, running the math.exs file defined above:

$ iex math.exs
iex> Math.zero?(0)
true
iex> fun = &Math.zero?/1
&Math.zero?/1
iex> is_function(fun)
true
iex> fun.(0)
true

Local or imported functions, like is_function/1, can be captured without the module:

iex> &is_function/1
&:erlang.is_function/1
iex> (&is_function/1).(fun)
true

Note the capture syntax can also be used as a shortcut for creating functions:

iex> fun = &(&1 + 1)
#Function<6.71889879/1 in :erl_eval.expr/5>
iex> fun.(1)
2

The &1 represents the first argument passed into the function. &(&1+1) above is exactly the same as fn x -> x + 1 end. The syntax above is useful for short function definitions.

If you want to capture a function from a module, you can do &Module.function():

iex> fun = &List.flatten(&1, &2)
&List.flatten/2
iex> fun.([1, [[2], 3]], [4, 5])
[1, 2, 3, 4, 5]

&List.flatten(&1, &2) is the same as writing fn(list, tail) -> List.flatten(list, tail) end which in this case is equivalent to &List.flatten/2. You can read more about the capture operator & in the Kernel.SpecialForms documentation.

Default arguments

Named functions in Elixir also support default arguments:

defmodule Concat do
  def join(a, b, sep \\ " ") do
    a <> sep <> b
  end
end

IO.puts Concat.join("Hello", "world")      #=> Hello world
IO.puts Concat.join("Hello", "world", "_") #=> Hello_world

Any expression is allowed to serve as a default value, but it won’t be evaluated during the function definition; it will simply be stored for later use. Every time the function is invoked and any of its default values have to be used, the expression for that default value will be evaluated:

defmodule DefaultTest do
  def dowork(x \\ IO.puts "hello") do
    x
  end
end
iex> DefaultTest.dowork
hello
:ok
iex> DefaultTest.dowork 123
123
iex> DefaultTest.dowork
hello
:ok

If a function with default values has multiple clauses, it is required to create a function head (without an actual body) for declaring defaults:

defmodule Concat do
  def join(a, b \\ nil, sep \\ " ")

  def join(a, b, _sep) when is_nil(b) do
    a
  end

  def join(a, b, sep) do
    a <> sep <> b
  end
end

IO.puts Concat.join("Hello", "world")      #=> Hello world
IO.puts Concat.join("Hello", "world", "_") #=> Hello_world
IO.puts Concat.join("Hello")               #=> Hello

When using default values, one must be careful to avoid overlapping function definitions. Consider the following example:

defmodule Concat do
  def join(a, b) do
    IO.puts "***First join"
    a <> b
  end

  def join(a, b, sep \\ " ") do
    IO.puts "***Second join"
    a <> sep <> b
  end
end

If we save the code above in a file named “concat.ex” and compile it, Elixir will emit the following warning:

concat.ex:7: warning: this clause cannot match because a previous clause at line 2 always matches

The compiler is telling us that invoking the join function with two arguments will always choose the first definition of join whereas the second one will only be invoked when three arguments are passed:

$ iex concat.exs
iex> Concat.join "Hello", "world"
***First join
"Helloworld"
iex> Concat.join "Hello", "world", "_"
***Second join
"Hello_world"

This finishes our short introduction to modules. In the next chapters, we will learn how to use named functions for recursion, explore Elixir lexical directives that can be used for importing functions from other modules and discuss module attributes.

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