Let’s handle file name extensions with Elixir, useful to validate the type of format of files that we would like to allow.

Step 1:

Let’s create a new Elixir project by going to the command line and typing the following in your desired directory:

$ mix new file_validator && cd file_validator

Step 2:

Open your test file test/file_validator_test.exs, remove the autogenerated test and add the following test:

  test "file validation should be false" do
    file = "somefile.exe"
    assert FileValidator.valid?(file) == false
  end

  test "file validation should be true" do
    file = "somefile.png"
    assert FileValidator.valid?(file) == true
  end

Go to the command line and run the test mix test

Step 3:

Let’s make our test pass by opening our main project file lib/file_validator.ex, remove the autogenerated hello world function and add the following:

  @extension_whitelist ~w(.jpg .jpeg .gif .png .pdf)

  def valid?(file) do
    file_extension = get_ext(file)

    @extension_whitelist
    |> Enum.member?(file_extension)
  end

  defp get_ext(file) do
    file
    |> Path.extname()
    |> String.downcase()
  end

Get back to the command line and run the test mix test now our test should pass with no failures.

Code Breakdown

@extension_whitelist ~w(.jpg .jpeg .gif .png .pdf)

The @ symbol means that is a module attribute, that can be used as a constant throughout our module.

The ~w sigil is used to generate lists of words(words are just regular strings). Inside the ~w sigil, words are separated by whitespace.

Path.extname(path)

Returns the extension of the last component of the path of a string.

Enum.member?(list, element)

Checks if an element exists within the list.

Conclusion

That’s a pretty simple and straightforward way to validate a file format, by getting the extension from the string with Path.extname(path) and passing it to Enum.member?(list, element) along with a whitelisted list of allowed file extensions. Thank you so much for your time, I really appreciate it.