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More about adding a collaboratorComposing 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
-
- Composing matchers with `change`
- Composing matchers with `contain_exactly`
- Composing matchers with `end_with`
- Composing matchers with `include`
- Composing matchers with `match`:
- Composing matchers with `output`
- Composing matchers with `raise_error`
- Composing matchers with `start_with`
- Composing matchers with `throw_symbol`
- Composing matchers with `yield_with_args`
- Composing matchers with `yield_successive_args`
- 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 toby
" 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 tofrom
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 over 7 years ago by myronmarston.