RSpec include matcherあるのにexclude matcherないのかと思ったら速攻作れた話

TL;DR

Negated Matchersを利用して、matcherを作成すれば終わる。

Exlude Matcherを作る(秒で終わる)

sample_spec.rb

RSpec::Matchers.define_negated_matcher :exclude, :include

リファレンスに普通に記載がある

Negated Matchers
Sometimes if you want to test for the opposite using a more descriptive name instead of using not_to, you can use Matchers.define_negated_matcher:

RSpec::Matchers.define_negated_matcher :exclude, :include
include(1, 2).description # => "include 1 and 2"
exclude(1, 2).description # => "exclude 1 and 2"
While the most obvious negated form may be to add a not_ prefix, the failure messages you get with that form can be confusing (e.g. "expected [actual] to not [verb], but did not"). We've found it works best to find a more positive name for the negated form, such as avoid_changing rather than not_change.

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