TIL: Elixir can pattern match at multiple depths in a single statement

One of Elixir’s greatest assets is its pattern matching. If you have ever used Elixir, you have probably had the pleasure of writing something like:

def background_check(%{manager: employee} = company) do
  %{name: full_name} = employee

  from(c in Criminals,
  where: c.full_name == ^full_name,
  select: c)
  |> Repo.one
  |> case do
    nil -> congratulate(employee)
    criminal -> notify(company)
  end
end

Here we are assigning the entire parameter map to a variable called company, and pattern matching to get the employee we want to do a background check on. We need to query our Criminals database table for a criminal with the same name as our employee. To do so, we first have to grab the name property off the employee object.

Well, today I learned, that you can have multiple matches in a single statement! With this newly acquired knowledge, we can simplify our background_check() function definition:

def check_company(%{manager: %{name: full_name} = employee} = company) do
  from(c in Criminals,
  where: c.full_name == ^full_name,
  select: c)
  |> Repo.one()
  |> case do
    nil -> congratulate(employee)
    criminal -> notify(company)
  end
end

Now we can pattern match to get the employee’s full_name, while also assigning the entire map under the manager key to the variable employee, as we did before.

Hopefully, you learned something too! Enjoy.