Issue #13583 has been updated by marcandre (Marc-Andre Lafortune).

Status changed from Closed to Open
Assignee set to matz (Yukihiro Matsumoto)

I'm not sure I like the current behavior of `transform_keys!`.

Two possibilities: `transform_keys!` is `each_key { delete(old_key), set(new_key) }` (as is currently) or `replace(transform_keys)` (I think I prefer this).

Matz, could you confirm what behavior you want?

Current:

    h = {1 => :hello, 2 => 'world'}
    h.transform_keys(&:succ) # => {2 => :hello, 3 => 'world'}
    h.transform_keys!(&:succ) # => {3 => :hello}

With using `replace`, we'd get the same results.

The current behavior allows partial updates though:

    h = {1 => :hello, 2 => :world}
    h.transform_keys! { |k| k == 1 ? :one : break }
    h # => {2 => world, :one => :hello}

With the `replace` version, `h` would be unchanged (or else we'd have to write an `ensure` to do the partial update)



----------------------------------------
Feature #13583: Adding `Hash#transform_keys` method
https://bugs.ruby-lang.org/issues/13583#change-68279

* Author: graywolf (Gray Wolf)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
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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