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RuboCop for
Code Review
Code Review Meetup #1
Jan. 19, 2018
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User.find(name: "pocke").inspect
● Masataka Kuwabara
● Engineer at Actcat, Inc. / SideCI
○ Rubyist, Gopher and Vimmer.
● RuboCop's core developer.
○ Today, I'll talk about RuboCop.
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Goal
● Improve code review with RuboCop.
● RuboCop is not a “Silver Bullet”.
○ When RuboCop reviews your code, you
should review RuboCop's warnings.
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Note
● I’ll talk about Ruby today, but we
can apply the practices to projects
that is written in other languages.
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Agenda
● What's RuboCop?
● The internal of RuboCop
● RuboCop for Code Review
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Questions
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Questions
● Are you Rubyist?
● Do you know RuboCop?
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What's RuboCop?
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What's RuboCop?
● A static analyzer for Ruby.
● It has 3 type checks.
○ Style
○ Lint
○ Metrics
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Style
Check coding style
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Style: check coding style
● White spaces
○ Indentation width
○ White spaces around brackets
● Syntax vs Method call
○ alias vs alias_method
○ for vs each
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Style checks based on one of the
style guides.
● https://github.com/bbatsov/ruby-
style-gudie
○ If your coding style has differences, you
can configure RuboCop.
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Style: example
# Use `def foo(arr)`
def foo arr
for x in arr # Use each
bar # Bad indentation
end
end
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Lint
Check bugs
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Lint: check bugs
● Unreachable code
● Debug code
● Unused variables
● etc...
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Lint: example
def foo
binding.irb # debug code
return true
# It’s never executed.
puts 'hi'
end
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Metrics
Check complexity
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Metrics: check complexity
● ABC size
○ Assignment, Branches, Conditional
● Line length
● Class length
● etc...
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The internal of RuboCop
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How does RuboCop check code?
● RuboCop analyzes each file
individually.
● RuboCop does not execute code.
See “Writing Lint for Ruby” in RubyKaigi 2017.
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Analyzing each
file individually
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Analyzing each file individually
● RuboCop does not check relations
between files.
○ e.g. db/schema.rb and models.
● For simplicity and performance.
○ RuboCop can analyze parallelly.
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Do not execute
code
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Do not execute code
● RuboCop doesn’t execute your
code.
○ Pros: If your code has system('rm -rf
/'), it does not remove.
○ Cons: RuboCop cannot understand
application completely.
■ e.g. eval, method call
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RuboCop for Code Review
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RuboCop for Code Review
● What does “RuboCop for Code
Review” mean?
● Advantage of RuboCop in Code
Review.
● Action for RuboCop Review
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What does
“RuboCop for
Code Review”
mean?
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What does “RuboCop for Code
Review” mean?
● RuboCop also reviews your code.
○ Human Review + RuboCop Review
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How does RuboCop review
● rubocop $(git diff --name-only)
before human review.
● I recommend using CI to run
RuboCop.
○ e.g. SideCI, CodeClimate and
Reviewdog.
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Advantages of
RuboCop in
review
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Advantages of RuboCop in
review
● Reduce human reviews.
● Enhancement reviews.
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Reduce human reviews
● Layout and style reviews by
human are no longer necessary.
○ e.g. “Remove the spaces”, “Use each
method instead of for”.
○ It’s a RuboCop job, not a human job.
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Enhancement reviews
● RuboCop can find bugs that
human will overlook.
○ Duplicate method definitions.
○ binding.irb(debug code)
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Action for
RuboCop Review
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Action for RuboCop Review
● You should review “RuboCop
Review”.
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Action for
RuboCop Review
For Style Reviews
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Action for Style Review
● Check your coding style.
○ If the review does not match with your
coding style, you should configure or
ignore the review.
○ http://rubocop.readthedocs.io/en/stabl
e/
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Action for
RuboCop Review
For Lint Reviews
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Action for Lint Review
● Lint reviews has false positives.
○ In other words, Lint review has a
mistake sometimes.
○ Because RuboCop does not know your
application completely.
■ See the slides above.
● It is not a bug, it is a restriction.
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How to start and/or
continue to use RuboCop?
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Start and/or continue to use
RuboCop
● You can get knowledges from
WEB+DB PRESS vol.102.
○ “RuboCopできれいで安全なコード”
written by me.
○ http://amzn.asia/fZ9cgZO
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Conclusion
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Conclusion
● RuboCop is not a “Silver Bullet”.
○ In many cases, the default coding style
doesn't match your coding style.
○ RuboCop has false positives.
○ So when RuboCop reviews your code,
you should review RuboCop Review.