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Defining a diffable matcher

When a matcher is defined as diffable, the output will include a diff of the submitted objects when the objects are more than simple primitives.

Define a diffable matcher (with diff-lcs 1.4)​

Given a file named "diffable_matcher_spec.rb" with:

RSpec::Matchers.define :be_just_like do |expected|
  match do |actual|
    actual == expected
  end

  diffable
end

RSpec.describe "two\nlines" do
  it { is_expected.to be_just_like("three\nlines") }
end

When I run rspec ./diffable_matcher_spec.rb

Then it should fail with:

       Diff:
       @@ -1 +1 @@
       -three
       +two

Define a diffable matcher (with diff-lcs 1.3)​

Given a file named "diffable_matcher_spec.rb" with:

RSpec::Matchers.define :be_just_like do |expected|
  match do |actual|
    actual == expected
  end

  diffable
end

RSpec.describe "two\nlines" do
  it { is_expected.to be_just_like("three\nlines") }
end

When I run rspec ./diffable_matcher_spec.rb

Then it should fail with:

       Diff:
       @@ -1,3 +1,3 @@
       -three
       +two
        lines

Redefine actual (with diff-lcs 1.4.4)​

Sometimes is necessary to overwrite actual to make diffing work, e.g. if actual is a name of a file you want to read from. For this to work you need to overwrite @actual in your matcher.

Given a file named "redefine_actual_matcher_spec.rb" with:

RSpec::Matchers.define :have_content do |expected|
  match do |actual|
    @actual = File.read(actual).chomp

    values_match? expected, @actual
  end

  diffable
end

RSpec.describe 'Compare files' do
  context 'when content is equal' do
    it { expect('data.txt').to have_content 'Data' }
  end

  context 'when files are different' do
    it { expect('data.txt').to have_content "No\nData\nhere" }
  end
end

And a file named "data.txt" with:

Data

When I run rspec ./redefine_actual_matcher_spec.rb --format documentation

Then the exit status should not be 0

And the output should contain:

2 examples, 1 failure

And the output should contain:

       @@ -1,4 +1,2 @@
       -No
        Data
       -here

Redefine actual (with diff-lcs 1.3)​

Given a file named "redefine_actual_matcher_spec.rb" with:

RSpec::Matchers.define :have_content do |expected|
  match do |actual|
    @actual = File.read(actual).chomp

    values_match? expected, @actual
  end

  diffable
end

RSpec.describe 'Compare files' do
  context 'when content is equal' do
    it { expect('data.txt').to have_content 'Data' }
  end

  context 'when files are different' do
    it { expect('data.txt').to have_content "No\nData\nhere" }
  end
end

And a file named "data.txt" with:

Data

When I run rspec ./redefine_actual_matcher_spec.rb --format documentation

Then the exit status should not be 0

And the output should contain:

2 examples, 1 failure

And the output should contain:

       @@ -1,4 +1,2 @@
       -No
        Data
       -here