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Project: RSpec Expectations 3.1

Publisher: RSpec

Composing Matchers

RSpec's matchers are designed to be composable so that you can combine them to express
the exact details of what you expect but nothing more. This can help you avoid writing
over-specified brittle specs, by using a matcher in place of an exact value to specify only
the essential aspects of what you expect.

The following matchers accept matchers as arguments:

  • change { }.by(matcher)
  • change { }.from(matcher).to(matcher)
  • contain_exactly(matcher, matcher, matcher)
  • end_with(matcher, matcher)
  • include(matcher, matcher)
  • include(:key => matcher, :other => matcher)
  • match(arbitrary_nested_structure_with_matchers)
  • output(matcher).to_stdout
  • output(matcher).to_stderr
  • raise_error(ErrorClass, matcher)
  • start_with(matcher, matcher)
  • throw_symbol(:sym, matcher)
  • yield_with_args(matcher, matcher)
  • yield_successive_args(matcher, matcher)

Note that many built-in matchers do not accept matcher arguments because they have precise
semantics that do not allow for a matcher argument. For example, equal(some_object) is
designed to pass only if the actual and expected arguments are references to the same
object. It would not make sense to support a matcher argument here.

All of RSpec's built-in matchers have one or more aliases that allow you to use a noun-phrase
rather than verb form since they read better as composed arguments. They also provide
customized failure output so that the failure message reads better as well.

A full list of these aliases is out of scope here, but here are some of the aliases used below:

  • be < 2 => a_value < 2
  • be > 2 => a_value > 2
  • be_an_instance_of => an_instance_of
  • be_within => a_value_within
  • contain_exactly => a_collection_containing_exactly
  • end_with => a_string_ending_with, ending_with
  • match => a_string_matching
  • start_with => a_string_starting_with

For a full list, see the API docs for the RSpec::Matchers module.

Scenarios
  1. Composing matchers with `change`
  2. Composing matchers with `contain_exactly`
  3. Composing matchers with `end_with`
  4. Composing matchers with `include`
  5. Composing matchers with `match`:
  6. Composing matchers with `output`
  7. Composing matchers with `raise_error`
  8. Composing matchers with `start_with`
  9. Composing matchers with `throw_symbol`
  10. Composing matchers with `yield_with_args`
  11. Composing matchers with `yield_successive_args`
  12. Composing matchers using a compound `and` expression
Composing matchers with `change`
Given
a file named "change_spec.rb" with: 
RSpec.describe "Passing matchers to change" do
  specify "you can pass a matcher to by" do
    k = 0
    expect { k += 1.05 }.to change { k }.
      by( a_value_within(0.1).of(1.0) )
  end

  specify "you can pass matchers to from and `to" do
    s = "food"
    expect { s = "barn" }.to change { s }.
      from( a_string_matching(/foo/) ).
      to( a_string_matching(/bar/) )
  end
end
When
I run rspec change_spec.rb
Then
the examples should all pass
Composing matchers with `contain_exactly`
Given
a file named "contain_exactly_spec.rb" with: 
RSpec.describe "Passing matchers to contain_exactly" do
  specify "you can pass matchers in place of exact values" do
    expect(["barn", 2.45]).to contain_exactly(
      a_value_within(0.1).of(2.5),
      a_string_starting_with("bar")
    )
  end
end
When
I run rspec contain_exactly_spec.rb
Then
the examples should all pass
Composing matchers with `end_with`
Given
a file named "end_with_spec.rb" with: 
RSpec.describe "Passing matchers to end_with" do
  specify "you can pass matchers in place of exact values" do
    expect(["barn", "food", 2.45]).to end_with(
      a_string_matching("foo"),
      a_value > 2
    )
  end
end
When
I run rspec end_with_spec.rb
Then
the examples should all pass
Composing matchers with `include`
Given
a file named "include_spec.rb" with: 
RSpec.describe "Passing matchers to contain_exactly" do
  specify "you can use matchers in place of array values" do
    expect(["barn", 2.45]).to include( a_string_starting_with("bar") )
  end

  specify "you can use matchers in place of hash values" do
    expect(:a => "food", :b => "good").to include(:a => a_string_matching(/foo/))
  end

  specify "you can use matchers in place of hash keys" do
    expect("food" => "is good").to include( a_string_matching(/foo/) )
  end
end
When
I run rspec include_spec.rb
Then
the examples should all pass
Composing matchers with `match`:
Given
a file named "match_spec.rb" with: 
RSpec.describe "Passing matchers to match" do
  specify "you can match nested data structures against matchers" do
    hash = {
      :a => {
        :b => ["foo", 5],
        :c => { :d => 2.05 }
      }
    }

    expect(hash).to match(
      :a => {
        :b => a_collection_containing_exactly(
          a_string_starting_with("f"),
          an_instance_of(Fixnum)
        ),
        :c => { :d => (a_value < 3) }
      }
    )
  end
end
When
I run rspec match_spec.rb
Then
the examples should all pass
Composing matchers with `output`
Given
a file named "output_spec.rb" with: 
RSpec.describe "Passing matchers to output" do
  specify "you can pass a matcher in place of the output (to_stdout)" do
    expect {
      print 'foo'
    }.to output(a_string_starting_with('f')).to_stdout
  end
  specify "you can pass a matcher in place of the output (to_stderr)" do
    expect {
      warn 'foo'
    }.to output(a_string_starting_with('f')).to_stderr
  end
end
When
I run rspec output_spec.rb
Then
the examples should all pass
Composing matchers with `raise_error`
Given
a file named "raise_error_spec.rb" with: 
RSpec.describe "Passing matchers to raise_error" do
  specify "you can pass a matcher in place of the message" do
    expect {
      raise RuntimeError, "this goes boom"
    }.to raise_error(RuntimeError, a_string_ending_with("boom"))
  end
end
When
I run rspec raise_error_spec.rb
Then
the examples should all pass
Composing matchers with `start_with`
Given
a file named "start_with_spec.rb" with: 
RSpec.describe "Passing matchers to start_with" do
  specify "you can pass matchers in place of exact values" do
    expect(["barn", "food", 2.45]).to start_with(
      a_string_matching("bar"),
      a_string_matching("foo")
    )
  end
end
When
I run rspec start_with_spec.rb
Then
the examples should all pass
Composing matchers with `throw_symbol`
Given
a file named "throw_symbol_spec.rb" with: 
RSpec.describe "Passing matchers to throw_symbol" do
  specify "you can pass a matcher in place of a throw arg" do
    expect {
      throw :pi, Math::PI
    }.to throw_symbol(:pi, a_value_within(0.01).of(3.14))
  end
end
When
I run rspec throw_symbol_spec.rb
Then
the examples should all pass
Composing matchers with `yield_with_args`
Given
a file named "yield_with_args_spec.rb" with: 
RSpec.describe "Passing matchers to yield_with_args" do
  specify "you can pass matchers in place of the args" do
    expect { |probe|
      "food".tap(&probe)
    }.to yield_with_args(a_string_matching(/foo/))
  end
end
When
I run rspec yield_with_args_spec.rb
Then
the examples should all pass
Composing matchers with `yield_successive_args`
Given
a file named "yield_successive_args_spec.rb" with: 
RSpec.describe "Passing matchers to yield_successive_args" do
  specify "you can pass matchers in place of the args" do
    expect { |probe|
      [1, 2, 3].each(&probe)
    }.to yield_successive_args(a_value < 2, 2, a_value > 2)
  end
end
When
I run rspec yield_successive_args_spec.rb
Then
the examples should all pass
Composing matchers using a compound `and` expression
Given
a file named "include_spec.rb" with: 
RSpec.describe "Passing a compound matcher expression to include" do
  example do
    expect(["food", "drink"]).to include( a_string_starting_with("f").and ending_with("d"))
  end
end
When
I run rspec include_spec.rb
Then
the examples should all pass

Last published almost 7 years ago by myronmarston.