[ruby-core:81304] [Ruby trunk Feature#13583] Adding `Hash#transform_keys` method

Issue #13583 has been updated by graywolf (Gray Wolf).


I don't think you missed anything, except I would just point out to also add `Hash#transform_keys!`. I don't know if it's worth mentioning or just kinda automatically assumed.

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Feature #13583: Adding `Hash#transform_keys` method
https://bugs.ruby-lang.org/issues/13583#change-64979

* Author: graywolf (Gray Wolf)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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In 2.4, new useful method `Hash#transform_values` was added. I would like to propose also adding matching method `Hash#transform_keys`.

```ruby
{a: 1, b: 2}.transform_keys { |k| k.to_s }
=> {"a"=>1, "b"=>2}
```

What needs to be considered is what to do in case of two keys mapping to the same new key

```ruby
{ a: 1, b: 2 }.transform_keys {|_| :same_key } # what should happen?
```

I think using `Hash[]` as model behaviour is a good idea.

```ruby
Hash[{ a: 1, b: 2 }.map { |key, value| [:s, value] }]
=> {:s=>2}
```

it's also how `Hash#transform_keys` works in rails (afaict).

This is a follow up feature request to #9970, which seems to be stalled. If the behaviour can be agreed upon, I can try putting together a patch (if no one else wants to step up).



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