View Source Modules and functions
In Elixir we group several functions into modules. We've already used many different modules in the previous chapters, such as the
iex> String.length("hello") 5
In order to create our own modules in Elixir, we use the
defmodule macro. The first letter of the module must be in uppercase. We use the
def macro to define functions in that module. The first letter of every function must be in lowercase (or underscore):
iex> defmodule Math do ...> def sum(a, b) do ...> a + b ...> end ...> end iex> Math.sum(1, 2) 3
In this chapter we will define our own modules, with different levels of complexity. As our examples get longer in size, 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.
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 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:
_build- contains compilation artifacts
lib- contains Elixir code (usually
test- contains tests (usually
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 and convenience purposes, Elixir also supports a scripting mode which is more flexible and does not generate any compiled artifacts.
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. This convention is followed by projects like
For instance, we can create a file called
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
Because we used
elixir instead of
elixirc, the module was compiled and loaded into memory, but no
.beam file was written to disk. In the following examples, we recommend you write your code into script files and execute them as shown above.
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)
The trailing question mark in
zero? means that this function returns a boolean. To learn more about the naming conventions for modules, function names, variables and more in Elixir, see Naming Conventions.
Giving an argument that does not match any of the clauses raises an error.
Similar to constructs like
if, function definitions support both
do-block syntax, as we learned in the previous chapter. 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-blocks for functions spanning multiple lines. If you prefer to be consistent, you can use
do-blocks throughout your codebase.
Function definitions 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. 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 \\ "hello") do x end end
iex> DefaultTest.dowork() "hello" iex> DefaultTest.dowork(123) 123 iex> DefaultTest.dowork() "hello"
If a function with default values has multiple clauses, it is required to create a function head (a function definition without a body) for declaring defaults:
defmodule Concat do # A function head declaring defaults 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 a variable is not used by a function or a clause, we add a leading underscore (
_) to its name to signal this intent. This rule is also covered in our Naming Conventions document.
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
Elixir will emit the following warning:
warning: this clause cannot match because a previous clause at line 2 always matches concat.ex:7: Concat
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.ex
iex> Concat.join "Hello", "world" ***First join "Helloworld"
iex> Concat.join "Hello", "world", "_" ***Second join "Hello_world"
Removing the default argument in this case will fix the warning.
This finishes our short introduction to modules. In the next chapters, we will learn how to use function definitions for recursion and later on explore more functionality related to modules.