quick_start

Quick Start Example

This is a working example demonstrating mutation testing with mutant.

This example is referenced in the main README.

Setup

bundle install

Run the tests

bundle exec rspec

Tests pass:

2 examples, 0 failures

Run mutant

bundle exec mutant run --use rspec --usage opensource --require ./lib/person 'Person#adult?'

Mutant finds surviving mutations:

@@ -1,3 +1,3 @@
 def adult?
-  @age >= 18
+  @age > 18
 end

The tests don't cover age == 18, so changing >= to > doesn't break them.

Fix it

Add a test for the boundary case in spec/person_spec.rb:

it 'returns true for age 18' do
  expect(Person.new(age: 18).adult?).to be(true)
end

Run mutant again - 100% coverage.

Compare coverage tools

Run the comparison script to see the difference between line coverage, branch coverage, and mutation coverage:

./compare_coverage

This runs:

1. SimpleCov (line coverage) - reports 100% because the line was executed
2. DeepCover (branch coverage) - reports 100% because both true/false branches were taken
3. Mutant (mutation coverage) - reports ~88% because mutations survive

Only mutation coverage catches that the boundary condition age == 18 is untested.

Note on mutation operators

This example demonstrates a simple operator replacement (>= to >). Mutant supports many more mutation operators including:

• Arithmetic operators (+, -, *, /)
• Logical operators (&&, ||)
• Comparison operators (==, !=, <, >, <=, >=)
• Bitwise operators (&, |, ^)
• Statement removal
• Boolean literal replacement
• Return value modification
• And many more...

See the meta directory for the full list of supported mutations.