In Passing One Matcher Into Another, you saw how RSpec defines a_value_within as an alias of the be_within matcher. It allows you to write expectations that read smoothly like the following one:
| | expect(results).to start_with a_value_within(0.1).of(Math::PI) |
You can use the same techniques in your own projects. Just call alias_matcher with the name of the new matcher first, followed by the existing one (the same order you’d use with Ruby’s alias_method):
| | RSpec::Matchers.alias_matcher :an_admin, :be_an_admin |
This snippet defines a new method, an_admin, which wraps the existing be_an_admin matcher (a dynamic predicate matcher that calls admin?; see Dynamic Predicates). The new matcher will use “an admin,” rather than “be an admin,” in its description and failure messages:
| | >> be_an_admin.description |
| | => "be an admin" |
| | >> an_admin.description |
| | => "an admin" |
The alias_matcher method can also take a block, for when you want something different from the matcher’s Ruby method name for your descriptions. For instance, if you wanted an_admin to show up as a superuser in the output:
| | >> an_admin.description |
| | => "a superuser" |
…you could define your alias like so:
| | RSpec::Matchers.alias_matcher :an_admin, :be_an_admin do |old_description| |
| | old_description.sub('be an admin', 'a superuser') |
| | end |
Dynamic predicate matchers like these are common targets for these kinds of aliases, since RSpec doesn’t ship with its own aliases for them.