Style cop idea: prefer map over each + <<

[vlad-pisanov]

I often encounter this pattern, especially amongst junior devs; they start with the [] literal, and then build up a list by repeatedly pushing elements into it. It's messy and inefficient.

# bad
result = []
arr.each { |x| result << x + 42 }

# good
result = arr.map { |x| x + 42 }

[natematykiewicz]

Is there a way to detect that multiple loops aren't pushing to a single array? Because I sometimes do this to cut down on array allocations. This cop would force me to use map instead of each, and then I'd have to concatenate the arrays together still, which results in N more array allocations and also N more object iterations (+ has to iterate the result list again during concatenation).

result = []
arr1.each { |x| result << x if something }
arr2.each { |x| result << x if something }

[ymap]

I've had some hesitations, but I decided to go ahead and create the PR. The following points were particularly complex. I would really appreciate any feedback.

• Given that Enumerable#map returns an Array, I limited the cop's reporting to cases with a statically determinable Array as the push destination.

• As previously noted, due to efficiency concerns, I restricted the cop's reporting to cases where the Array being pushed to was clearly initialized as empty. Of course, using += for non-empty Array was considered, but I feel it's more appropriate as a specific option in the cop, if needed.

• For cases where each's return value might be used, I skipped autocorrection. Although rare, prioritizing safety over simplicity seemed crucial.