Test behavior and boundaries

Use setup data, doctests, focused assertions, and async-safe tests.

01 · Start with the whole example

Run this first

Run it once, change one input, and compare the new result.

Elixir

# lib/score.ex
defmodule Score do
  def pass?(score) when is_integer(score) and score in 60..100, do: true
  def pass?(_score), do: false

  def check(score) when is_integer(score) and score in 0..100,
    do: {:ok, score}

  def check(_score), do: {:error, :out_of_range}
end

# test/score_test.exs
defmodule ScoreTest do
  use ExUnit.Case, async: true

  test "pass boundary" do
    assert Score.pass?(60)
    refute Score.pass?(59)
  end
end

Check the result first

mix test reports one passing test containing two boundary assertions.

02 · Take the code apart

Read from the first line down

Read use ExUnit.Case, async: true: Allow independent tests to run concurrently.
Read setup: Return context data for each test.
Read doctest Module: Run documented IEx examples as tests.

03 · Meet the new symbols

Symbols are not secret signs

use ExUnit.Case, async: true

Allow independent tests to run concurrently.

setup

Return context data for each test.

doctest Module

Run documented IEx examples as tests.

04 · Ideas inside the code

Match each name to its meaning

test boundary

Boundary cases sit where behavior changes, such as 59 versus 60.

setup

A setup callback prepares fresh data for each test.

doctest

Doctest runs examples written in module documentation.

05 · Make the idea clear

Why these forms are useful

Run the example first. Predict one result, then change one input and run it again.

Boundary cases sit where behavior changes, such as 59 versus 60.

A setup callback prepares fresh data for each test.

06 · Change it yourself

Close the answer and try

Add a test that rejects a negative score.

Practice starting point

test "negative score" do
  assert Score.check(-1) == ____
end

Target result: Expect {:error, :out_of_range}.

Stuck? Read one hint

Use the tagged error returned by the public API.

After you run it, see one answer

One answer

test "negative score" do
  assert Score.check(-1) == {:error, :out_of_range}
end

Think about it: When is `async: true` unsafe?

Avoid it when tests share mutable external state such as one database record or global process.

Take with you

Remember these three lines

1
Test behavior, not private steps.
2
Place tests on both sides of boundaries.
3
Share examples through setup or doctests only when they stay clear.

Lesson completeRun the code once, then mark this lesson as done.