Use index_by and index_with wherever possible

[eugeneius]

Using index_by or index_with is more concise than each_with_object and more performant than map { ... }.to_h or Hash[map { ... }].

I found these by writing a RuboCop rule, which I've submitted to the RuboCop Rails project in rubocop/rubocop-rails#208.

[kaspth]

Dig this! They really are more concise. I’d be curious if Ruby might accept them in core 😄

[rafaelfranca]

I had to wrap my head to understand what was going on with those method. Even the documentation of index_with is not clear enough.

[kaspth]

True, they do take a little bit of learning. I find it quite understandable now though and the benefits far outweigh that initial cost. My mind sees it as index means hash from Enumerable, then by means derive the keys and with means derive the values. Haven’t looked at the index_with documentation since I wrote it, true it really only clicks once you get index_by, and there was contention around the naming of course. Again, I could live with that because it’s better than what was before.

[eugeneius]

Upstreaming index_by was proposed in https://bugs.ruby-lang.org/issues/8490, but rejected; Matz didn't like that in this case "index" means "key" when elsewhere it refers to an array offset.

Enumerable#associate, which is effectively index_with, was proposed seven years ago in https://bugs.ruby-lang.org/issues/7341 but has never received a response from Matz.

Maybe a new proposal would be more successful, but it seems unlikely that the current names would be accepted. I'll think about some alternatives.

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@rafaelfranca do you find the index_by changes less clear, or just the index_with ones? I remember that I found index_by intuitive the first time I saw it, but index_with took a while.

I've tried to clarify the docs for both methods in #38677.

[rafaelfranca]

I find the concept of Array.index_* resulting in a Hash confusing. But the docs clarify it.

[koic]

@eugeneius FYI, RuboCop Rails 2.5.0 including rubocop/rubocop-rails#208 has been released. Thank you for your great code!
https://github.com/rubocop-hq/rubocop-rails/releases/tag/v2.5.0

[eugeneius]

Thanks for the heads up @koic 😊 I've opened #38992 to propose enabling the new cops.