[new cop] Require parentheses

[cbarraford]

Rubocop, prefers the use of && over and. This isn't just a syntactic change, it also changes how if statements are evaluated. In the example below, notice in way one, the include? statement has a space then tuesday. While, way two uses parentheses instead of a space.

#!/usr/bin/env ruby
# ruby 2.0

weekdays = %w[
  sunday
  monday
  tuesday
  wednesday
  thursday
  friday
  saturday
]

# way 1
puts 'way one'
if weekdays.include? 'tuesday' && true == true
  puts 'its true'
else
  puts 'its false'
end

puts
# way 2
puts 'way two'
if weekdays.include?('tuesday') && true == true
  puts 'its true'
else
  puts 'its false'
end

Output

way one
its false

way two
its true

As you can see, the outcome of the if statement is different, and its not immediately obvious why. Furthermore this failure, fails silently which is even worse.

I propose a new cop that enforces the use of parentheses for cases such as this. Thoughts?

[jonas054]

I think that's an excellent idea. There's a rule in the Style Guide that says

Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that have "keyword" status in Ruby (e.g. attr_reader, puts) and attribute access methods. Use parentheses around the arguments of all other method invocations.

And I think we've stayed away from a cop implementation of it because it's so difficult to know what's a DSL and what isn't. But the more limited case of semantically problematic omission of parentheses might be easier to implement a cop for, a lint cop.

[bbatsov]

Yeah, that sounds reasonable. I guess we can also have some cop checking for parens used in built-in Ruby DSL methods (like attr_, puts, p, etc), as we obviously know them.

[mattwhite]

Or just enforce parantheses when the line contains one of: and, or, not, &&, ||, !

The order of operations can be tricky to remember, especially if you also use python.

[claco]

👍 on this one. I would love to have this.