Issue #16989 has been updated by knu (Akinori MUSHA).


I don't like the idea of making `array & hash` possible, as that would go against the trends of making Ruby type-friendly and error-prone.

akr (Akira Tanaka) wrote in #note-25:
> - 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.

I'd appreciate the list as implementation details, but if it were like some hashes are used as sets, programs might get less readable.
For example, a program that goes `hash = {}; hash << 1; hash[:key] = :value` would likely be a mistake, but that couldn't be easily detected without explicit type annotation.

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Feature #16989: Sets: need 
https://bugs.ruby-lang.org/issues/16989#change-89895

* 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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