[ruby-core:121327] [Ruby master Feature#16993] Sets: from hash keys using Hash#key_set
From:
"mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date:
2025-03-13 11:16:43 UTC
List:
ruby-core #121327
Issue #16993 has been updated by mame (Yusuke Endoh).
This was discussed at the dev meeting. @matz wanted to hear more about the use cases for this method first.
Also, since Set is currently in a halfway state where it is not fully built-in, a built-in `Hash#key_set` might require a special hack like `Enumerable#to_set`.
https://github.com/ruby/ruby/blob/7c88cbb4a6c486348c44be24941f17ef8be6b329/prelude.rb#L34
@akr pointed out the possible consistency issue of `Hash#compare_by_identity`. Since `Hash#transform_values` keeps this `compare_by_identity` flag, the proposed patch will assign a Hash with `compare_by_identity` flag to `@hash` in Set. We will need to review it carefully to make sure it does not break the implicit assumptions of `set.rb`.
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Feature #16993: Sets: from hash keys using Hash#key_set
https://bugs.ruby-lang.org/issues/16993#change-112294
* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
----------------------------------------
To create a set from hash keys currently implies a temporary array for all keys, rehashing all those keys and rebuilding a hash. Instead, the hash could be copied and its values set to `true`.
```ruby
h = {a: 1}
# Now:
Set.new(h.keys) # => Set[:a]
# After
h.key_set # => Set[:a], efficiently.
```
--
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