This article was originally published on Elixir Recipes.

While Elixir is dynamically typed, we can supply optional typespecs to make sure our programs are type-safe. Preventing type errors ensures that there’s no discrepancy between differing types of our program’s constants, variables, and functions. Typespecs are also useful as documentation.

If you’ve used type systems in OCaml or F#, Elixir’s type system should feel familiar to you.

Here’s an example:

defmodule Bob do
  @spec say(String.t) :: String.t
  def say(message) do
    "Bob says: #{message}"
  end
end

We use @spec to specify the type of the input and its return value in the following format:

@spec <method name>(<type of parameter>) :: <type of return value>

In the example above, our method say takes in a string and returns a string.

Elixir provides many built-in types. You can also declare your own custom domain-specific types that can then be re-used in other modules.

What happens if the implementation doesn’t match the spec? Let’s modify our code:

defmodule Bob do
  @spec say(String.t) :: String.t
  def say(message) do
    "Bob says: #{message}"
    123 # The return type is now integer instead of String
  end
end

Typespecs are not used by the compiler, so if you compile and run this code, you’ll receive no warnings and the code will run. To perform type checking, we use Dialyzer.

You can set up Dialyzer by installing dialyxir globally, and using the generated plt as your default dialyzer_plt.

Let’s perform some type checking:

$ elixirc bob.exs # Compiles to Elixir.Bob.beam
$ dialyzer Elixir.Bob.beam
  Checking whether the PLT /Users/yosriady/.dialyzer_plt is up-to-date... yes
  Proceeding with analysis...
typespecs.exs:2: Invalid type specification for function 'Elixir.Bob':say/1. The success typing is (_ ) -> 123
 done in 0m1.03s
done (warnings were emitted)

Nice! Dialyzer caught our type error. We can fix it and check again:

$ elixirc bob.exs
$ dialyzer Elixir.Bob.beam
  Checking whether the PLT /Users/yosriady/.dialyzer_plt is up-to-date... yes
  Proceeding with analysis... done in 0m0.93s
done (passed successfully)

For Mix projects, running dialyxir on our project’s root directory will perform automatic compilation of our Elixir code. We can also set up git hooks to ensure that all type violations are fixed prior to making/pushing commits and reap the full benefits of static analysis.

In Closing

Elixir gives you the expressivity of a dynamic language and the type checking capabilities of statically typed language through typespecs. Static typing is a powerful tool to express what you know about your problem domain into your code, preventing type errors.

Additional reading:

• Typespecs