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Improve PredicateMatcher recommendations #2068

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095025e
Use map lookup in PredicateMatcher
bquorning Mar 29, 2025
411875d
Improve PredicateMatcher recommendations
bquorning Mar 29, 2025
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1 change: 1 addition & 0 deletions CHANGELOG.md
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Expand Up @@ -5,6 +5,7 @@
- Fix false positive in `RSpec/Pending`, where it would mark the default block `it` as an offense. ([@bquorning])
- Fix issue when `Style/ContextWording` is configured with a Prefix being interpreted as a boolean, like `on`. ([@sakuro])
- Add new `RSpec/IncludeExamples` cop to enforce using `it_behaves_like` over `include_examples`. ([@dvandersluis])
- Fix `RSpec/PredicateMatcher` sometimes suggesting the `be_key` matcher instead of `have_key`. ([@bquorning])

## 3.5.0 (2025-02-16)

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55 changes: 33 additions & 22 deletions lib/rubocop/cop/rspec/predicate_matcher.rb
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Expand Up @@ -69,18 +69,22 @@ def message_inflected(predicate)
matcher_name: to_predicate_matcher(predicate.method_name))
end

TO_PREDICATE_MATCHER_MAP = {
exist?: 'exist',
exists?: 'exist',
include?: 'include',
instance_of?: 'be_an_instance_of',
is_a?: 'be_a',
key?: 'have_key',
respond_to?: 'respond_to'
}.freeze
private_constant :TO_PREDICATE_MATCHER_MAP

def to_predicate_matcher(name)
case name = name.to_s
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Side note, unrelated to this PR. This list is incomplete, since RSpec has support for eg start_with/end_with. They work with Arrays, too. Also cover/exist/...

when 'is_a?'
'be_a'
when 'instance_of?'
'be_an_instance_of'
when 'include?', 'respond_to?'
name[0..-2]
when 'exist?', 'exists?'
'exist'
when /\Ahas_/
name.sub('has_', 'have_')[0..-2]
if TO_PREDICATE_MATCHER_MAP.key?(name)
TO_PREDICATE_MATCHER_MAP.fetch(name)
elsif name.start_with?('has_')
name.to_s.sub('has_', 'have_')[0..-2]
else
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How do you feel if we raise an offence, but don't autocorrect the "else" cases?

"be_#{name[0..-2]}"
end
Expand Down Expand Up @@ -241,18 +245,25 @@ def move_predicate(corrector, actual, matcher, block_child)
corrector.insert_after(actual, ".#{predicate}" + args + block)
end

TO_PREDICATE_METHOD_MAP = {
a_kind_of: 'is_a?',
an_instance_of: 'instance_of?',
be_a: 'is_a?',
be_a_kind_of: 'is_a?',
be_an: 'is_a?',
be_an_instance_of: 'instance_of?',
be_instance_of: 'instance_of?',
be_kind_of: 'is_a?',
include: 'include?',
respond_to: 'respond_to?'
}.freeze
private_constant :TO_PREDICATE_METHOD_MAP

def to_predicate_method(matcher)
case matcher = matcher.to_s
when 'be_a', 'be_an', 'be_a_kind_of', 'a_kind_of', 'be_kind_of'
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Subjectively, this reads better than multiple hash items

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I changed it to a hash lookup because one of the Metric cops said the method became too long. But I agree with you, and I will change it back when/if I get back to this PR.

'is_a?'
when 'be_an_instance_of', 'be_instance_of', 'an_instance_of'
'instance_of?'
when 'include'
'include?'
when 'respond_to'
'respond_to?'
when /\Ahave_(.+)/
"has_#{Regexp.last_match(1)}?"
if TO_PREDICATE_METHOD_MAP.key?(matcher)
TO_PREDICATE_METHOD_MAP.fetch(matcher)
elsif (subject = matcher[/\Ahave_(.+)/, 1])
"has_#{subject}?"
else
"#{matcher[/\Abe_(.+)/, 1]}?"
end
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3 changes: 3 additions & 0 deletions spec/rubocop/cop/rspec/predicate_matcher_spec.rb
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Expand Up @@ -91,6 +91,8 @@
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `be_something` matcher over `something?`.
expect(foo.has_key?('foo')).to be_truthy
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `have_key` matcher over `has_key?`.
expect(foo.key?('foo')).to be_truthy
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `have_key` matcher over `key?`.
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Does this imply that the receiver is an instance of a Hash?
If it’s e.g. an SqlColumn, have_key? won’t fit, and it should be is_key? instead

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Can we make a distinction at least if there’s an argument?

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TIL key? is aliased as has_key? https://ruby-doc.org/3.4.1/Hash.html#method-i-include-3F

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Proposal:
Only auto-correct expect(foo.key?(foo)) with a single argument.
Only raise an offence with zero or multiple args or a block

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@bquorning bquorning Mar 29, 2025 •
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To be honest, I don't really like this cop (or others) that make broad assumptions about types which can not safely be deduced with static analysis. There will always be false positives.

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@bquorning bquorning Mar 29, 2025 •
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TIL key? is aliased as has_key?

And I thought one of them had been deprecated, but apparently that's not the case.

I also think this method is aliased as member? and include? btw.

expect(foo.is_a?(Array)).to be_truthy
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `be_a` matcher over `is_a?`.
expect(foo.instance_of?(Array)).to be_truthy
Expand All @@ -103,6 +105,7 @@
expect(foo).to be_something('foo', 'bar')
expect(foo).to be_something 1, 2
expect(foo).to have_key('foo')
expect(foo).to have_key('foo')
expect(foo).to be_a(Array)
expect(foo).to be_an_instance_of(Array)
RUBY
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