composing_matchers.feature

Feature: 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.

  Scenario: Composing matchers with `change`
    Given a file named "change_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `contain_exactly`
    Given a file named "contain_exactly_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `end_with`
    Given a file named "end_with_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `include`
    Given a file named "include_spec.rb" with:
      """ruby
      RSpec.describe "Passing matchers to `include`" 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

  Scenario: Composing matchers with `match`:
    Given a file named "match_spec.rb" with:
      """ruby
      RSpec.describe "Passing matchers to `match`" do
        specify "you can match nested data structures against matchers" do
          hash = {
            :a => {
              :b => ["foo", 5.0],
              :c => { :d => 2.05 }
            }
          }

          expect(hash).to match(
            :a => {
              :b => a_collection_containing_exactly(
                a_string_starting_with("f"),
                an_instance_of(Float)
              ),
              :c => { :d => (a_value < 3) }
            }
          )
        end
      end
      """
    When I run `rspec match_spec.rb`
    Then the examples should all pass

  Scenario: Composing matchers with `output`
    Given a file named "output_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `raise_error`
    Given a file named "raise_error_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `start_with`
    Given a file named "start_with_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `throw_symbol`
    Given a file named "throw_symbol_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `yield_with_args`
    Given a file named "yield_with_args_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers with `yield_successive_args`
    Given a file named "yield_successive_args_spec.rb" with:
      """ruby
      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

  Scenario: Composing matchers using a compound `and` expression
    Given a file named "include_spec.rb" with:
      """ruby
      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