each_with_object style must respect immutable method_args

[abriel]

each_with_object does not work on immutable objects like integer.
http://stackoverflow.com/questions/19064209/how-is-each-with-object-supposed-to-work

In case of
[1, 2, 3].reduce(0) { |a, e| a += e }
rubocop must leave it as is.

rubocop -V
0.34.2 (using Parser 2.2.2.5, running on ruby 2.2.2 x86_64-linux)

Thanks.

[minustehbare]

Can you provide the message you are receiving? I reproduced the environment you listed and used a test file (shown below) and got a warning which does not relate to each_with_object:

W: Useless assignment to variable - a. Use just operator +.

puts [1, 2, 3].reduce(0) { |a, e| a += e }

[abriel]

r = [1, 2, 3].reduce(0) do |memo, item|
  memo += item > 2 ? item : 0
  memo
end

puts r

Offenses:

t.rb:1:15: C: Use each_with_object instead of reduce.
r = [1, 2, 3].reduce(0) do |memo, item|
              ^^^^^^

1 file inspected, 1 offense detected

If I just replace like it suggested, I got:

t.rb:1:5: W: The argument to each_with_object can not be immutable.
r = [1, 2, 3].each_with_object(0) do |item, memo|
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected

Which is right.

[abriel]

There is the simple condition inside the closure, but in real it could be complex and cannot be modified into a simple one.

[minustehbare]

Ok, with that example I am getting the same output; thank you. I toyed around with different syntax and it seems that RuboCop::Cop::Style::EachWithObject class specifically checks if your inject or reduce block ends with the accumulator value you pass in (e.g. memo).

# this does not generate an offense
r = [1, 2, 3].reduce(0) do |memo, item|
  memo + item > 2 ? memo + item : memo
end

I was also able to reproduce this error against the latest master changes using Ruby 2.2.3 and Parser 2.2.3.0

rubocop -V
0.34.2 (using Parser 2.2.3.0, running on ruby 2.2.3 x86_64-linux)

I will look into providing a fix in a pull request.

[alexdowad]

The point isn't whether the initial value for reduce is immutable or not. Replacing reduce with each_with_object is not valid if the accumulator variable is assigned to within the block.

I'm just opening a PR which fixes this in EachWithObject.