Testing on the Elixir track

Learn how to test your Elixir exercises on Exercism

From the terminal, change to the base directory of the exercise then execute the tests with:

$ mix test

This will execute the test file found in the test subfolder -- a file ending in _test.exs

Pending tests

In test suites of practice exercises, all but the first test have been tagged to be skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

If you wish to run all tests at once, you can include all skipped test by using the --include flag on the mix test command:

$ mix test --include pending

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

Additional Elixir testing features

ExUnit and mix test provide quite a few methods of grouping, tagging & executing tests and various methods of controlling test execution, much of which is summarized below.

Methods for running specific tests


Running tests in a specific file

All tests in a single file may be executed via mix test by specifying the file:

$ mix test test/<FILE>.exs

NOTE: tagging may impact which tests are actually executed using this method.

Running individual tests

Individual tests may be executed by referencing a test's line number in the file:

$ mix test test/<FILE>.exs:LINENUM

Multiple tests may be executed by giving multiple line numbers separated by :.

For example, given a file with the following content with line numbers:

test "Test 1" do           # 1
  # test implementation    # 2-6
end                        # 7
                           # 8
test "Test 2" do           # 9
  # test implementation    # 10-21
end                        # 22
                           # 23
test "Test 3" do           # 24
  # test implementation    # 25-35
end                        # 36

The 1st and 3rd tests can be executed by:

$ mix test test/FILE.exs:1:24

NOTE: When specifying tests via line numbers, tagging is ignored.

Running groups of tests

Tests may be grouped using describe:

describe "short test group description" do
  test "test description" do
    # test implementation

  test "another test description" do
    # test implementation

All tests in a group may be executed by referencing its line number in the file, just like referencing and executing individual tests.


Other useful mix test options

  • --include and --exclude - runs or doesn't run specific tests based on their @tags
  • --failed - runs only tests that failed the last time they ran
  • --max-failures - the suite stops evaluating tests when this number of test failures is reached
  • --seed - seeds the random number generator used to randomize the order of tests --seed 0 disables randomization so the tests in a single file will always be ran in the same order they were defined in
  • --stale - runs only tests which reference modules that changed since the last time tests were ran with --stale


Typespecs and Dialyzer (DIscrepancy AnalYZer for ERlang programs)

Elixir exercises include a skeleton implementation file in the lib subdirectory. This file outlines the module and functions that you are expected to implement. In most exercises, you will find typespecs above the function declaration. These start with the @spec tag and typically follow the @spec function_name(type1, type2) :: return_type format. These are used in Elixir and Erlang as documentation and in conjunction with a tool called Dialyzer to find type inconsistencies and possible bugs. For more information see the Elixir typespecs guide or the typespecs documentation. For documentation about Dialyzer see Erlang -- dialyzer.

Optionally, you may want to check the types of your implementation with Dialyzer. There are a couple of steps you will need to take. In order to do that, you must add the Dialyxir dependency to the mix.exs file for your problem.

defp deps do
  [{:dialyxir, "~> 0.4", only: [:dev]}] # <-- Add this

Then use mix tasks to get and compile from the command line:

$ mix deps.get
$ mix deps.compile

If this is the first time you have run Dialyzer you will most likely not have a plt file. The persistent lookup table, or PLT is used by Dialyzer to cache information about built in Elixir and Erlang types. To create a plt with sensible defaults run:

$ mix dialyzer --plt

Finally it can be run with:

$ mix dialyzer

Make sure to change the path to your system's path to the Elixir libraries. For instance, if you installed Elixir with homebrew you will probably find it under /usr/local/Cellar/elixir/1.3.2.

It should be reiterated that running Dialyzer and removing all warnings is an optional step when completing an exercise. Dialyzer warnings can be difficult to decipher. For instance, examine the warnings for this very silly implementation of the Bob exercise.

defmodule Bob do
  @spec hey(input :: String.t) :: String.t
  def hey(input) do

  def hey(input) do

This produces the following warnings.

bob.exs:2: Invalid type specification for function 'Elixir.Bob':hey/1. The success typing is (_) -> 1
bob.exs:7: The variable _input@1 can never match since previous clauses completely covered the type any()

The first warning means that the function is not returning the correct type. The last indicates that the second function definition can never be reached because the first function definition always matches.