Issue #16989 has been updated by akr (Akira Tanaka).


knu (Akinori MUSHA) wrote in #note-23:
> akr (Akira Tanaka) wrote in #note-22:
> > I like extending Hash instead of incorporating Set.
> > For example, #inspect omit hash values.
> > (Of course, modifying Hash#inspect is clearly not acceptable, I think some compromise is possible)
> 
> How would you implement methods and operators of Set in Hash?  Would they only work for hashes that meet special conditions?

- define a special constant (Hash::DummyValue = Object.new)
- Hash#inspect doesn't show the value: {1 => Hash::DummyValue}.inspect #=> "{1}"
- `{}` can be used as an empty set
- `hash << v` would be defined as `hash[v] = Hash::DummyValue`
- Hash#each doesn't work well as Set but we can use Hash#each_key.
- Hash#to_a doesn't work well as Set but we can use Hash#keys.


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Feature #16989: Sets: need 
https://bugs.ruby-lang.org/issues/16989#change-89888

* Author: marcandre (Marc-Andre Lafortune)
* Status: Assigned
* Priority: Normal
* Assignee: knu (Akinori MUSHA)
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I am opening a series of feature requests on `Set`, all of them based on this usecase.

The main usecase I have in mind is my recent experience with `RuboCop`. I noticed a big number of frozen arrays being used only to later call `include?` on them. This is `O(n)` instead of `O(1)`.

Trying to convert them to `Set`s causes major compatibility issues, as well as very frustrating situations and some cases that would make them much less efficient.

Because of these incompatibilities, `RuboCop` is in the process of using a custom class based on `Array` with optimized `include?` and `===`. `RuboCop` runs multiple checks on Ruby code. Those checks are called cops. `RuboCop` performance is (IMO) pretty bad and some cops  currently are in `O(n^2)` where n is the size of the code being inspected. Even given these extremely inefficient cops, optimizing the 100+ such arrays (most of which are quite small btw) gave a 5% speed boost.

RuboCop PRs for reference: https://github.com/rubocop-hq/rubocop-ast/pull/29
https://github.com/rubocop-hq/rubocop/pull/8133

My experience tells me that there are many other opportunities to use `Set`s that are missed because `Set`s are not builtin, not known enough and have no shorthand notation.

In this issue I'd like to concentrate the discussion on the following request: `Set`s should be core objects, in the same way that `Complex` were not and are now. Some of the upcoming feature requests would be easier (or only possible) to implement were `Set`s builtin.



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